FBM,
It's odd that you would not be happy to fly as aircraft captain in the right seat with your friend flying the aeroplane in the left but you would be happy sitting in that seat as a passenger being flown by him as an out of date and out of practice pilot. He may be the captain in the latter case but it will still hurts if he gets it wrong!
If you're not comfortable to fly as captain in the right seat then that is a problem but, if that's the case you wouldn't be of much use going along to "help" your friend anyway. A "helper" in this situation (with a pilot who is out of practice) would, I suggest have to be ready and able to interceede including taking over control if nessecary.
Sounds like the best solution for your friend is to find an instructor who is familiar with the peculiarities of the aeroplane in question. What sort of aeroplane are we discussing ayway; how peciliar is it?
Happy landings
3 Point