Your wife and your friend have always gotten along really well, so you agree that it does make a lot of sense for her to be the one to try to bring him out of his funk.
I’ve picked out or bought lingerie for her to wear him many times…
She was too embarrassed to admit it last night...
She's right. You DID know the reason -- or, at least, you were pretty sure that was the reason. And since there's nothing you can do about it, you know you are just going to have to accept that being cuckolded is a part of your relationship with her.