Lawsuit - isn't remotely the answer. In fact it will probably make the situation worse.
Does she gets the grades she deserves. Its important to know if this professors bias enters the grading. You said no females got an A on the labs, but the better question is, should they have. You can sometimes check with other professors to see what they would have given for grades on a lab. If she got a B and deserved a B, there is no sexual discrimination when grading. The other questions would be if he extends past grading and to behavior, more than comments, treats you less, different treatment from male and female students. More than just comments, actions.
Also just because there hasn't been obvious changes (not sure what you would look for), doesn't mean that the Department Head isn't trying to remedy the situation. People take a while to change. The Department Head could be checking to see if its just more than your gf.
When I was in college, I had a girl accuse a Professor of a similar thing, (although he didn't make comments that girls cannot do engineering). Turns out there was none, the girl just had alot of problems in the class, and through the course of blaming everyone but herself, she was convinced he was discriminating against women. I'm not saying that this is your case, just saying why sometimes, it takes time for things to be checked out.
Also, if you want to go one step up. Talk to a dean or school administrator outside of the department. They are usually the best people to talk to when you want something done. I'd be very surprised if they didn't act.