I would offer to let her buy the one back maybe? Of if you feel like you can, just give him back. He will not bring much more than likely anyway.
If the one you don't want is sold, where do you think he will realistically end up? If he has issues, depending on what they are, and at his age, I imagine his end will not be a good one.
If you are okay with finding out later that he went to slaughter, then go ahead and sell him if you want, and your wife might as well kiss that friend goodbye.
Had you all not taken the horses, and I am sure that the prior owner thought they would be staying together, she might have found someone else who would have kept them together and to their natural demise.
In the end, it is up to you, and what you can live with.
Horses make me a better person.