Everyone has private conversations that would be hurtful for their partners to overhear. Everyone. Everyone needs to be able to just vent sometimes. The conversation that you read was her first impression that she was surely exaggerating for comedic value to her friends. You guys are young and likely preoccupied with that sort of thing.

Second of all, her friend sucks. She’s jealous, petty, trying to ruin the relationship intentionally and no friend at all. You need to tell your girlfriend that. You need to genuine explain to her with all sincerity how reading that made you feel. Hopefully she will understand, reassure you, apologize, and never judge someone so harshly again. This will be a learning experience for her. You guys are gonna have a lot of these moments in your 20s. When I was in my 20s, I learned the most about how my actions and words truly affect others. What she did was wrong and thoughtless but if your relationship is solid then I think you can find it in you to be understanding of this and move on. But if you just can’t let the hurt go, I would also understand if you decided to move on.

Good luck