I don't see anything wrong with your situation. Your friend would rather fly on the more expensive airline (better food, cuter FA's, for whatever reason), but cannot afford the difference in price. You want them to have a great trip so you offer to give them the $$ difference so they can take the airline of thier choice. There is no requirement for them to "give" you the FF ticket that results from them flying on their preferred airline, but since you are family, if in the future they knew you needed an award ticket, I am sure they wouldn't hesitate to provide you with a FF ticket.