Yeah.. this is a hard situation.

But you know in the end, at his age, you can't demand he stop seeing her. He needs to learn that relationship lesson on his own.. and this fighting and arguing now althogh it probably will not last, should certainly teach him what he doesn't want in a relationship.

I think the most you can do is keep an open dialogue with him about relationships and what constitutes a healthy relationship and things like that. I wouldn't trash the gf in front of him AT ALL. This will only lead to him resenting you in some way, which is going to push him away and make him close up his ears.. You could say something like "gee, you know im really concerned over all the yelling and arguing you two do, is there anything you want to discuss that maybe i could help you figure out how to handle the fighting better?" or whatnot..

In the end, because I know you are a good mom. I know that he will make the right choice for him.. its just a matter of time