I'm going to pretend this is a real thing.
Unless his wife, your wife, or some woman was there, calling it a bi-experience is poor salesmanship. If that makes it easy for him, fine, but nobody else will be buying. There's a reason Catholics make such a big deal about secret confessions. It stays between you and God and whoever your priest tells. It does not test the faith of your spouse and your friends, who now have the burden of "forgive others as I have forgiven you."
That's really your only argument in favor of carrying this secret to his grave. It's an act of selfishness, not an act of contrition. It dumps his guilt on everybody else and accomplishes nothing but make him feel better. In any case, why does your name have to come up? It's one thing to confess ones own sins, but why drag others into it? It's unlikely you are the only one.
Beyond that, never discount the law of intended consequences. Your friend may want out of his marriage, but doesn't have the intestinal fortitude to say it plainly and then walk out. He may see this confession and the expected fallout to relieve him of this relationship, along with a few others. He'll act as if this is his just punishment for his transgression, but it was what he really wanted all along.
Adulterous secrets are seldom as secret as we think they are. His wife may suspect, or actually know about you, or some other man. As long as this secret is kept in a limited circle, she can abide with it. Once it becomes public, she'll have to act, one way or the other.
