I would send that text message to her from your phone while she's sitting next yo you and tell her to start talking. Then, after she tells her washed down version. I would call the friend and tell her that gf told you everything and you want to confirm with her exactly who was with man 1 and 2. Hopefully, you get their names.

Then, if the relationship is over, you make an exit plan. It is workable you find a therapist to help navigate the road to trusting again