My friend can't stand my girlfriend and I don't want to stand up for her when he says what he thinks about her
You shouldn't let people badmouth her though. Do you think your girlfriend did something wrong?
Yes. I'm afraid of what she did to him behind my back
If your friend had a real bad experience with your girlfriend, you should hear him out. You can learn new information about him and your girlfriend.
Nah... He's just looking for trouble. I'm going to try and avoid this quarrel.
I think trying to stay out of the argument is a good choice. Taking a side between the two will inevitably lead to one of them being very upset with you. Letting them work it out on their own is probably a good way to go here.