When I found out he'd been staying there (he was leaving one morning when I dropped off the kids.....we have an agreement that nobody either of us date is allowed around the kids until we know that it's going to be a serious relationship.) about a month ago, I asked her if things were getting serious. She said "we haven't slept together or anything like that.....but I can say we aren't just friends."
There were so may responses I could have made to that that would have been damaging. But I said "it's none of my business. You two are adults and you're human as much as I am. What you guys do is up to you."
And yet there are so many responses you could have made that might've been a little less . . . validating? I would've just said "Thank you for letting me know," or "Thank you for being honest with me."