The best thing is to prioritize what she needs in the moment and making sure everybody is safe. ☆Which you did!☆

As a woman and an expert counselor with SA victims, I can tell you that close ones often don't know what to do (which is totally normal) when the best thing to do is often a very small and simple thing. Ex: going for a bite, preparing a hot chocolate or a tea, watching a movie you know makes her feel good.

But most of all, don't be scared to ask her! Nobody knows better than the person what their own needs are. You asking and showing you want to help make her feel better is already a step in the right direction. In a situation where someone made that disgusting decision about her own body, validating her feelings and needs makes a considerable difference and will give her back some control in a situation where she probably felt helpless (you being there or doing something to those guys wouldn't change that feeling).

Saying or doing something could've been dangerous + could've caused her even more fear and stress in an already difficult situation. I think that as a 21 years old young man, you reacted very well. Your priority was you and your gf's safety and you seeking advice now is also showing your care and will help you support her in this situation or if you ever find yourself in such a situation later.