<human>: Can you give me the gist of the text in a nutshell?

I recently broke up with my girlfriend (she's polyamorous) in a very serious relationship even though it didn't last for very long and she didn't take it very well, at least that's what I'm getting. One of my close friends is in a long distance relationship, and I'm not sure what he is, but a year ago I heard he doesn't mind the polyamorous thing. 

Since we broke up they've been hanging out together in her dorm alone, and he might be tonight at the time of writing. I'm a paranoid person and while it isn't my problem anymore (technically) it hurts at the possibility of a really close friend (possibly) cheating on his girlfriend and being with my ex, he might not see anything wrong with it (which is my problem) and it just seems inconsiderate. 

It seems rude to me that he would do that (if he is), which makes me just rationalize in my head that he isn't, but if he is, should I care about it that much? I feel like it's natural right after a break up to be bothered by your ex being with someone so soon, and I'm monogamous (which is only a tiny reason for the break up, but I wish not to go into it) so the viewpoints are very different, but I would think my close friend would have some "post breakup etiquette" at least.
<bot>: Close friend might be sleeping / making sexual advances on my ex girlfriend (I broke up with less than a month ago from time of writing from a serious relationship) and I don't know how I should react to the situation.