What are you trying to achieve?
From where I am standing, if you want to try break them up and get with them, DON'T. You will probably lose all respect that you have with your mate (Even if he is an ass) and the girl may pick up on it and lose respect/trust as well. I have a friend who is a complete dick to his girlfriend and I confronted him once telling him about how I felt with him treating her like that. He changed not too long afterward, for the better. You may want to consider going this to your mate?
If you want to just be there for her, then by all means go ahead and do that, I would probably recommend it. By showing that you are someone she can lean on away from her boyfriend will build trust and a close friendship. But at the same time, try not to ruin your mates relationship.
If you would like to clarify your question it would make things easier.
