I'm going to come with my girlfriend to a meeting with her boss
What do you think will happen if you go to her meeting too?
I'm going to help her win a raise.
I understand that you want to help your gf get more money, however, going to a meeting with her and her boss to discuss her compensation makes her look weak. Stay out of your gf's work life and mind your own business.
I'm going to complain that she gets too little cash.
I think that it is okay to feel that way but you should let her deal with that on her own. It would be odd to accompany her to a meeting at her workplace.