Name changed for this.
Happened to be on Facebook earlier today and cousin's husband had shared (accidentally?) a comment he'd made on a Facebook dating site. He deleted it within the hour, so I'm guessing he either realised he'd screwed up or somebody else warned him.
The comment was along the lines of 'when you look that good, who cares', it wasn't personal, I don't think he knew the person he was commenting on but regardless of that it's still a bit/lot of a red flag.
I've opted to say nothing to my cousin because I don't know how the Facebook dating site works, can you only comment if you're a dating site member or, if it happens to show up on your feed, can anyone make a comment?
Either way her husband is an idiot, but I don't want to accuse him of being on dating sites/cheating if it was just a one-off, stupid, inappropriate comment.
YABU - yes, you need to let her know
YANBU - to say nothing