It depends on the thickness of the leather you typically use for your saddle. I have seen Bison skirting which was 13 oz in the butt and around 7oz in the shoulder. So yes you could make Bison for a saddle, but you have to have enough hides to sort or enough leather to sort for thickness. Also something to consider, bison have terrible cutting yields due to holes and scratches in the leather. If your friend does not mind this and his hides are big enough than yes it should work if you have a tannery willing to do the work.
