I believe I understand your concerns...

Mainly, I think you don't want the fact that you are the buyer to open up a can of worms so-to-speak. You don't want her to turn around and say that you guys took her for a ride, and either throw a tantrum or try to sue you/use it as some sort of ammo.

Trust me, though, if these are indeed your concerns, following the advice outlined will provide you and he with very limited liability.

Be smart about the transaction, be above-board, do not try to finagle the price, as long as you do nothing wrong, there is really no legal recourse she has for simply being angry that you were the one to buy the vehicle.

Are there any pieces missing?