My wife was actually in this situation a few years back! A single mutual friend of both my wife and the wife in question was on a dating site and found the husband in there... Told my wife like "OMG what do I do??", and they did some digging to make sure it was actually him (not a fake profile). The friend who found him decided to steer well clear of it... My wife (after weeks of deliberating) decided to tell her
. Talk about awkward. Hahaha. They weren't really close friends or anything, more just through having young kids, but yeah... Cringe... Heh. She confronted her husband and they're still together so who knows what happened there. Other than still being on a couple of the same FB groups her and my wife aren't really friends any more, but that's as much to do with lives going in different directions as anything.
Personally? I doubt it was worth it, I think I'd just leave it well alone unless it was a VERY close friend. Maybe try to orchestrate a situation where they "see for themselves" rather than be the one to tell them
.