Well, Ok I'll see if maybe I can help you a bit further. On the point where you say that she doesn't want you to say anything to them, let me ask you this. Does she know these two guys enough to be able to conclude that 1: At least one of them is a good friend of yours and 2: Can she maybe already know that this guy isn't going to apologize, even if asked? If the answer is yes to those two questions, then it's possible that she simply doesn't want to "start something", and be the reason why there would be tention and hostility between you and your friend. She cares about you, so she doesn't want you to loose a friend because of a situation involving her. Is that a possibility?
You kind of hint at your girlfriend maybe not having the best selfconfidence. You kind of describe her as a bit withdrawn or shy maybe? And it sounds like she has difficulty talking about how she feels. That's something you need to work on. It might be that she has had bad things happen in the past, but it might also be because you've reacted in a negative way when she's tried. I'm not saying you have, that's something for you to consider tho. You seem like you have the tendency to get very angry over things pretty easily. Your go-to emotion when faced with something that is negative or confisuing seem to be anger. Maybe she tried to talk to you about something that was bothering her before, and you reacted by getting angry? I have no clue if that's happened, but it's not uncommon for people (who already are shy or not talkative to begin with) to clam up if something like that has happened.
Either way, your friends are not that important right now. But I'll just mention it a bit before telling you what you should probably do to deal with this. Next time you hang out with your friend, just when you get a chanse, bring it up with him. Don't do it as a confrontation, just calmly say "What happened that night at the bar can't happen again. I won't tolerate it." and then talk to him about it. Hold firm on that though. What happened between them 10 years ago is irrelevant and he has to drop it.
The important bit is that I think you should really, really think about who you are as a boyfriend. What are your flaws, just from your perspective in this situation, what have you done that may have contributed to the negatives in your relationship. If you were a woman, would you want to be in a relationship with you as you are now? Why, and why not? Take a couple of hours and just reflect upon yourself, and your relationship, and based off of that, what are your faults? What are your girlfriends faults? What do you want to fix? Once you know that, talk to your girlfriend, in a calm way where you frame it as "I love you and I want this relationship to work" and then you two work on it.
I'm sorry, but you may have to choose who is going to be more important to you for your future, between your friend and your girlfriend. Honestly, your friend doesn't seem like that great of a guy. He sounds immature, spitefull and like he can hold a grudge over stupid bullshit like nobodys business. Your girlfriend doesn't sound perfect, but she doesn't sound like a bad person. It might be that you're holding on to this friendship because you've been friends for so long, but it might be that you're outgrowing these guys. Just think about it, and talk to your girlfriend, and good luck. Sorry for the leantgh of this.