Generally though, you know that if you fuck someone that was with a friend or family member (Especially if it is very quickly after a breakup) that it is going to involve some drama (likely trying to still seperate feelings), and you generally have considerations for family/friends feelings. Not considering their feelings at all, does make you a little bit of an asshole. In the very least, not a very good friend or trustworthy/caring family member.

It's acceptable in that it's not cheating. But it sure doesn't show much compassion/empathy/care for your family member.....

Furthermore this is a 31 year old man, having sex with a girl still in highschool, that is fucked up in of itself. He then also gets back with this girls aunt never mentioning this tidbit... The guy at the very least is a major asshole scumball.