Sorry you're going through this ((Xelora)). Is your friend a man or woman? First I don't think you should apologize to her - if anything she should. I think you should stick up for yourself and your friend should also set the GF straight, that she shouldn't diss you. Your friend should be saying something to the GF.

I agree it sounds like she's jealous and feels threatened. She needs to deal with her own insecurities. Your friend needs to tell the GF to back off and not interfere in your friendship. She needs to be civil and learn tolerance.