OP here; thanks, all, for the good discussion, especially after the swift early rush of “MYOB.” (Not to say that that crowd is wrong, but it makes me feel less crazy to hear that other people understand where I was initially coming from.)
To try to answer some of the things…
My friend did not explicitly ask me for advice, though we’re close enough and have exchanged unsolicited advice freely enough over the time we’ve been friends that there was a tacit acceptance when he filled me in that I was welcome to comment. I have, however, tried to move carefully - beyond, as mentioned, the initial discussion with him about disease, emotional entanglements, and the pain what he’s doing will cause his wife.
He has not asked me to cover for him, and I interact so seldom with his wife that I don’t think it would come up. When I said I was not friends with her that’s not to say I don’t know her; I do, but she is often away from home for long periods for work, and when I do see her…well, we just don’t click. Nothing terrible about her, and my lukewarm response to her is irrelevant to what my friend is doing to her.
Why am I friends with him, and will I remain friends with him? It’s hard to say. Terrible though I think what he’s doing is, it’s also hard to totally dismiss all the many good qualities I’ve seen in him. Stupid though this may sound, he’s been a very kind and - yes - loyal friend. If I were ever in trouble or stranded somewhere in the middle of the night, he’s the person whose number I’d call. In so many ways he’s a smart, funny, thoughtful guy.
That does not diminish the terribleness of what he’s doing to his wife, and in spite of his good qualities, the moment he told me I confess that I became a bit wary of him: I’m his friend, but she’s his wife, and if she can’t trust him, what chance do I have? I can’t immediately “unfriend” him, but I might have to slowly start the process.
Part of why I think he should just end his marriage is not just because it would be kinder to his wife (again, he’s made clear that he’s not going to stop cheating), but because I suspect that he would also be happier. I think he loves his wife, but he also doesn’t seem to mind terribly when she’s away. He seems born to be a single guy, and his behavior - having flings here and there - would not be my style of things, but it would be fine if he were single. When he told me what he’d been up to it was in the same kind of tone that someone with a drinking problem might tell you that they just know they’re never going to sober up: as though they’d faced their demons, struggled with them, been defeated time and time again, and knew that it was just a part of themselves that they couldn’t excise. Could he stop cheating? Yes, he absolutely could - and should - in the same way that that alcoholic should put down the bottle. But if he knows that he just won’t do it, I hope he either tells his wife or splits up with her.
He’s sure his wife doesn’t know, and he’s also sure that she would be very upset if she found out. But maybe she does know - or at least suspect it - and maybe she’d be more fine than he expects if he told her. Who knows - people come to all sorts of understandings. Maybe I should just encourage him to come clean to her, and if that leads to divorce, well, at least she knows; but maybe she’ll say that as long as he comes home at night to her she’s fine with whatever else he wants to do.