1) At the very least you want the face that will be riding your bandsaw's face or pivot bar to be perfectly flat. However, you really also want the opposite to have also been planed flat and parallel to the opposite face so that you get a consistent thickness from one end to the other while resawing. Otherwise the thickness of the off cut piece could vary significantly if the starting board was not planed to a consistent thickness over its length. For example, if you only jointed one side of a bowed board with a 1/4" bow then each end of your off cut piece might be 1/2" thick but the middle might only be 1/4" thick, which might render it unusable spending upon what you were expecting to get out of the cut.
2) Following #1, you do ideally want the starting board jointed on one face and then planed parallel to that face to ensure you get a consistent thickness when you resaw. Otherwise thickness may vary considerably over the length of the board. In fact, ideally you re-plane the face of the source timber prior to each resaw operation so that you always have one clean face on the off cut piece and to remove any waviness in the previous resaw operation as one seldom makes a perfectly smooth resaw cut from one end of a long board to another (particularly not without a bit of practice since you often have to keep steering the board along the way to maintain a consistent thickness since the blade will sometimes try to follow the grain or may have a drift angle to it, hence why pivot bars are often mounted to a fence for resawing longer boards as they permit the operator to continually adjust the blade's path.
3) It is highly desirable to joint at least the one edge that rides the table as you want to be able to feed the timber smoothly through the blade without it hanging on the leading edge of the table. The last thing you want to have to do is fight with a timber (especially if a large and heavy timber) that keeps getting hung up on the table.
4) The degree of board imperfections will greatly influence your choice in terms of resawn thickness as will your project's tolerance for a board that is not perfectly flat, as can the wood's original cut type (flatsawn is much more likely to cup than rift or quartersawn, for example). The more defects you start out with and/or the more concern you have that the wood may change its shape after resawing due to the sudden change in humidity that the formerly interior wood is now exposed to as well as the degree to which a perfectly flat board matters to your project the more excess material you will want to leave on each cut to allow for later jointing and planing operations. In fact, after resawing it is best to wait a few days to a week, or more, after resawing before jointing and planing the new bords to allow the new boards to acclimate to your shop -- especially if you do not control the humidity level in your shop year round (I keep my shop between 40-50% year round with a dehumidifier to reduce wood movement, rusting, and to make for a more comfortable shop) -- especially for flatsawn lumber as such is highly prone to movement due to its grain and ring orientation (the end grain often tells a board's story and how it is likely to react to changes in moisture).
5) The best guideline is to leave as much excess as practical and the thicker the board you desire the more you will typically want to leave. But this is more an art than any hard and fast rule and you will have to learn when the rules may be broken to a degree and when they must not (it is always better to have extra wood on hand because wood is an imperfect medium even if you seemingly do everything right).
In your case you will have a board that starts out somewhere between 1-1/8" and 1-5/16" since it is 5/4 lumber, and you want to get 12/16", or 3/4", of material out of that board. But remember that we have not accounted for thickness lost to jointing and planing the starting timber flat and parallel, the kerf of the resaw blade (and any error in your resawing, which there will be some), plus you will need enough remaining material to possibly rejoint each new board if they undergo movement in the following days plus the needed thickness to then plane them flat and parallel. Whether you can get the two pieces you seek out of the one board you will start with will depend entirely upon the aforementioned knowns and unknowns -- you might be able to and you might not depending upon how perfect the board both starts out as (in other words, if little material needs to be removed the more likely you are, and vice versa), how perfect your resaw cut is and how wide the blade kerf, followed by how much the wood may move afterwards.
You stand a decent chance of getting the two boards you desire provided the final jointed and planed source timber is as close to a true 5/4" (1-1/4") as practical, but if it gets anywhere near 1" then you are taking a big risk trying to get 3/4" of finished boards with only 1/4" of sacrificial material for the resaw kerf and jointing and planing operations as you might find yourself with less than 3/32" of leeway to work with assuming a 1/16" blade kerf, and that is very risky for flatsawn lumber and/or an inexperienced operator.
Typically, for boards under 1/4" I prefer to leave at last 1/8" of added thickness to work with, around 3/16" for 1/2" thick boards, and at least 1/4", or more, for boards 3/4" and thicker to ensure adequate material remains for later planing and jointing operations. I also like to wait a few days if possible to allow any final acclimating even though my shop is humidity controlled, non humidity controlled shops may wish to wait a week or two before planing to final dimensions, planing equal amounts off each face of the board.
But it really is more art than hard rules as there are times when you may be able to cut things more closely and times when you should not, and it is not always clear ahead of time how much you can push things until after the resawing is done...so it is best to err on the side of leaving more and avoiding leaving too little sacrificial thickness unless you have plenty of wood to spare. Still, sometimes it is best not to resaw at all as you may find yourself with one, or more, of the boards being too thin for your project if you try to get them all out of one board to begin with.
On a related note, most typical 14" bandsaws are really not setup well for resawing wide boards and even their original 6" height can sometimes be a bit optimistic for any real amount of resawing (hence risers are not really an ideal upgrade if one has resawing in mind as they are already underpowered with the typical 3/4HP motor). Resawing takes a good bit of tension and requires a good bit of power out of the motor due to the amount of teeth in continual contact with the wood and the amount of waste material that needs to be continually removed. You can get through the resaw operation but it can be very slow going and tedious depending upon the wood species and you might get some burning or burnishing of the wood if sawing slows down too much. You generally want a fair bit of tension when resawing wide boards to avoid barrel cuts but not all 14" bandsaws can safely deliver the required tension without either twisting of the main column (especially with added riser installed) or potential damage to the upper wheel tracking adjustment mechanism, which is often just a simple pot metal piece prone to catastrophic failure if the tension is ever dialed too high (Zinc is a strong but bery brittle metal and behaves a lot like glass under stress). So if you want to do much resawing in the future then you may wish to look into a larger bandsaw for resawing and use the 14" bandsaw for those jobs it most excels at (they are really the "jack of all trades" bandsaw and well suited to many other tasks, just not resawing wide lumber whereas larger bandsaws excel at resawing but are not good "jack of all trades" bandsaws). But if resawing is something you expect to do very seldom then you may well get by with your existing bandsaw, particularly if you can avoid the harder woods that are more demanding.
Good luck! I wish you all the best as the ability to process large lumber into thinner dimensions as needed greatly expands your opportunities and allows one the option of buying much less expensive wood in the rough -- it can also be a lot of fun IMHO. Just make sure your friend knows that resawing is not a guaranteed outcome if the permitted buffer thickness is not sufficient and can potentially result in two undersized boards in such cases, but if the jointed and planed thickness of the starting lumber is close to 1-1/4" then you stand a good chance, but anything much less, and especially if it gets anywhere close to 1", then the operation is much higher "risk" due to an overly tight budget, you might get want you want and you might not! So long as he knows that going in then use both of your best judgement as to whether to proceed.
