Not irrelevant at all, and live-in certainly means nothing. What if he looks like a walrus? Just the fact she's live-in doesn't answer up's question.
As to the topic of the OP, I'll jump in the fray, but go the other direction. It's none of your business what arrangements other people have. If both of them are ok with what he did, then great. If not, well, I guess he'll be having a "blue" vacation. Either way, the problem will resolve itself.
Please explain why what she looks like has to do with my original post. And no, I don't understand why his looking like a walrus would matter.
My original post said they were debating it, and thus obviously both of them are not ok with that he did. Otherwise they would not be debating whether what he did was right, would not have brought it to my attention and asked my opinion, and I would not have posted here. Thanks for telling me I'm butting my business into other people's supposed arrangements and then adding your 2 cents, of course. I always find it amusing when a poster flames the OP without carefully reading the original post - thanks so much for your kind words!