I don't think spending time with is girlfriend will enrich his social life. He hangs out with her most of his time, and he the time they do spend together they are not enjoying it. Also by spending time with her, he is avoiding time with your other friends. If he hangs out with others, he will have a greater chance of meeting new people and start to rebuild his social network.
OP sorry to say it, but this relationship is going sour. If you don't end it soon, she will, those little arguements will quickly build up into bigger ones until it's over. End it on YOUR terms, so you can have the dignity of knowing she didn't dump you. And how, just sit her down one day, some place quiet and public and tell her it is not working out and you want to end it. Obviously there is no real nice way of ending it, so just be quick and clean with the cut.