You know, a lot of times I get letters like this: "Hi, hi! I have a friend and I think I have a little crush on him. By the way, he seems to be flirting with me, too. Yeah, yeah, we chat with him all the time. But there is one problem… He already has a girlfriend. I think she's just awful and totally wrong for him. But he keeps going out with her for some reason. What should I do??". That's why I've decided that you and I have to work it out, finally. And if you're in the same situation, study the manual "What to do if you hate his girlfriend.
Don't get involved
Step aside. Otherwise you'll end up being a very bad girl anyway and you won't win anything. You tell him to break up with her? He'll tell you it's none of your business, which makes sense. And what's more, he'll probably tell you that you just don't know her. And he'll be right.
Respect his choice
Seriously, I'm telling you. Whether you like it or not, you have to respect the other person's choice. Even if it's a guy you like.
Don't try to break them up
Becoming the cause of their breakup is not the best thing that could happen to you. Especially if you're friends with him. And even more so if you want to go out with him later. Let things happen without you being involved. Then the chances of you having a relationship in the future will increase.
Stop communicating with him if just being friends doesn't suit you
The main thing you have to remember is that this guy is busy. And even if he's flirting with you, it doesn't mean he's ready to break up with his girlfriend. If you can't just be friends with him, forget about him.