Need advice

My best friend (of four plus years) slept in the bed with me and a guy I was talking to, because there was no where else for her to sleep. I trusted her. I learned that they snuck away while I was asleep and had sex. When I found out I confronted her. She told me she didn't mean for it to happen. She wasn't under the influence. I don't understand how you just accidentally let that happen. I was starting to like him a lot and she knew that. Idk how to deal with this. This is something I never expected. She wants my forgiveness but I don't know that I have it in me. I don't know if I could forgive her if I wanted. Things won't ever be the same.. I could use some good advice