Well it’s shitty of you to do that. Frankly a much worse thing to do than her being kissed by a stranger against her will. This was a deliberate act of breaking the trust on your part.

Have you considered that she was trying to protect your feelings and didn’t think she could discuss this with her friend because it’s a personal story that belongs to you two only? She might felt she needs to sort it out with you first. Next, in her mind the bad thing was someone trying to force themselves on her, so him kissing her was the lesser of the two evils. So she shared the really bad and scary thing that could have happened but thankfully didn’t.

Your girlfriend got harassed and assaulted from the way it sounds. I’d give her the benefit of the doubt until she’s back and you can talk through it. Be supportive. It was likely a traumatizing experience and she needs to know you aren’t going on a victim blaming trip here.