I wasn't able to find a way to find a way to word my issue properly in the title because of limited space and lacking grasp on the English language -- so I'm sorry if the title might be somewhat misleading, I'll explain my situation here in full:

It's not about jealousy in the traditional sense - I have utter and complete trust in my girlfriend's faithfulness, and I'm not saying this to convince you or myself. It's about how I am afraid that my lack of action when she will be hit on in a couple of days would be perceived as weak.

See, there's this guy that fooled around with my girl a couple of years ago. He is very much a part of her extended circles, and she shut him out for half a year in the past after he made some condescending remarks about her choice of boyfriend (even I can tell that this is worded awkwardly, but bear with me). I was somewhat upset at the time because things were still fresh and he made no secret of the fact that he considers himself a better choice for her, even going as far as insulting me publicly but indirectly (I didn't know him at the time). This was at a stage at which I didn't know how she ticked in regards to those situations. I pleasantly watched her react by blocking him on facebook because she considered it unacceptable of him to behave that way towards her boyfriend. Now, while it is no secret that he wants to fuck her silly, he did apologize to her profusely some months after, which I found great, because for all the trouble his bursting libido might cause, they still share common ground as friends, and I'm all for everybody being friends with everybody. Good vibes 'n shit. Anyway, as I said, I'm not worried about her cheating, I am worried about how my reaction (or lack thereof) to someone disrespecting our shared personal space would reflect on how our more and more overlapping circles of friends perceive me. As confident as I might be in what we have, and as little of a shit I might give about someone gnawing their teeth off fruitlessly, I am rather insecure of my position in group compositions I don't yet feel at total ease with.

I don't even know if his apology was sincere. He might try to hit on her, as he did before (she showed me those messages for a laugh, unprompted), and she will firmly tell him to fuck off -- but I feel like I should at least ridicule him for the attempt.. or.. I don't fucking now.. shower him in kindness and telling him that I know about where his dick is pointed, and then proceeding to treat him like anyone else at the rave we're going to.

Jesus, what a ramble. I usually deal with any unfamiliar social situation by just winging it, but this one stumps me. Ignore him? Acknowledge him? Mock him? Befriend him? Throw a punch if he oversteps his boundaries?

I've been successfully faking confidence, masculinty and worldliness before when my default levels didn't cut it (not gonna pretend like I don't have my lows), but I dread the situation of facing that guy and can't talk myself out of this insecurity. I am not used to be given any shit by guys because of my presence and stature alone, but I feel like this might change when I encounter that guy.

How do I prepare myself mentally? Can someone share similar experiences? Am I making myself crazy for nothing?

edit: I just realized that I misrepresented the situation slightly - I am not just worried how the group might perceive me, I'm also unsure as to what my girlfriend might expect in such a situation. Imagine he hits on her, overtly disrespects me.. am I a caveman for reacting aggressively (mainly verbally)? Should I play it cool?

Sorry for the unorganized wall of text, but this is making my head explode, I'm not used to insecurity anymore. I feel like I'm a teenager again.