<human>: So this past weekend, a friend of mine threw a party and invited "John" and his girlfriend "Caitlyn" as well as myself and a sizable group of other friends. Some in our group are close with Caitlyn and John, and I've known them for a while, but I'm not in contact with either of them outside of these big group gatherings.

Caitlyn's always been really friendly, but this summer she's ramped it up to overt flirting: becoming very animated when I'm around, reaching forward for my elbow mid-conversation - that sort of thing. Usually I find it flattering and don't do anything to stop it.

This weekend I get to this house party (I'm one of the last to arrive), and she notices me immediately. From the couch, she calls out my name until she has my undivided attention, pats down a spot next to her for me to sit, and calls me over. I sit down. I just got a drastic haircut, so she complements it a few times over the course of the conversation. She mentions that her T-shirt dress looks "kind of slutty". I get noticably uncomfortable and the wind goes out of her sails pretty quickly. Meanwhile, John is on his tablet on the other side of her, not saying a word. Eventually I get up and leave (the couch, not the party).

Later that night, I tell John that I'd like a chance to talk to him, but the chance never comes up. I don't really know what to say. I feel guilty about not bringing it up earlier with him, or confronting her, or just doing something useful about it. I'm not sure what my next move should be. I can get in contact with either of them, but I'm not sure it would help in any way and I might end up just embarrassing myself.

Provide a brief overview of the text.
<bot>: Should I tell an acquaintance that his girlfriend has been flirting with me? If so, what's the best way to do it?