Your girlfriend is right. I could understand if you were picking an expensive restaurant to maybe treat your friends out once per year and you know they could not afford it otherwise, but still want to share the experience. But just because you make more money is not reasonable for you to be covering every bill. Even broke college students know if they are going out they need to cover their tab. There is no reason for your friends not to cover theirs.

That said. If you have known them for a while maybe it is good to talk. Have you sort of assumed you were covering the tab and have never really allowed them a chance to cover their parts or do they expect you to cover the tab. Because depending the answer to that you will get two different answers. Because if it is the later, yeah they have been abusing your kindness. But if it is the former I think a simply conversation can solve this.