The GM is responsible for the game foremost, which does not mean that the players should not make it easy on them. You suggest the fact that the GM is the problem player's girlfriend is irrelevant. But if that player is favored by the GM that fact is probably part of the problem. People have a tendency to favor their close ones. It is totally possible that she doesn't even notice and that it happens unconsciously. You should talk to the GM, preferably alone, and clearly but nicely communicate the problems. You should tell her it's normal that her games are not perfect as she has just started her GM career. And you should provide counsel or feedback. If you have GM experience, providing some counsel on what might work better might be very helpful for her. In any case you should make sure that your input is not perceived as a personal reproach. It is in the responsibility of the GM to know, look up and enforce rules. Although, ideally the player should make this easy for the GM as well by knowing or looking up and respecting the rules themselves that are associated to their character. If you think that the GM does not do this enough,you should tell them so while making sure that you do not sound accusatory. Looking up rules yourself is also a valid option. If you do not want to disrupt the gamebor to be perceived as a rules lawyer, you should tell the respective player and / or the GM about rules problems outside of the game. When I run a game, I try to know all necessary rules which mostly works but I also make it clear that players can and should indicate potential problems in the cases where it does not work. You should also tell your friend that you are not having much fun with his playstyle even if it is no longer a homebrewed built.