The first thing that comes to mind reading your post is the proverbial pecking order, which exists in our human interactions as surely as it does in nature. Have you ever watched a bunch of baby chicks grow from little peepers to mature hens? The dominance of the future hen "leaders" happens before the others even realize what's happening. And all of a sudden, they are the poor little ones being picked (pecked) on. Entering a new work environment can be like that. If you don't figure out the group dynamics and establish rapport with others it's hard to procure a good spot for yourself in the "order". Or at least ensure you don't become the scapegoat for the incompetence of others.
To your girlfriend, it seems like it should be good enough to just do your job and be recognized for a day of work well done. And I agree that should be sufficient, but it is unfortunately quite contrary to reality when others sense in you either a lack of confidence or authority. Part of what will help her move up the "pecking order" where she's not ignored or taken advantage of is:
1.) Make solid eye contact with others when speaking to them. Smile.
2.) I would guess your GF is soft-spoken - she needs to speak louder and get comfortable with it, with the sound of her voice. It's OK to be heard.
3.) Take an assertiveness training course. People subconsciously know what the human "pecking order" is and we carry that around in our daily lives. If she were to gain some extra confidence it would help all her personal interactions.
4.) You've rightly pointed out that human interactions are part of the issue here - as much as it might seem difficult or distasteful, your GF needs to watch others and figure out the dynamics of her workplace. You would be an excellent sounding board for this no doubt. Then, you can help her develop relationships and avoid future bad situations.
5.) Take a martial arts class - becoming physically strong enhances your mental strength too.
Ultimately, people will treat you as well or poorly as you give them permission to. I hope these thoughts help - and I send along to help soothe the frustration of the moment.