So me (18M) and my gf (18F) have been dating for over a year now. A couple days ago she was out with her friends and my best friend drinking. After the night ended she and my best friend went home together as they live close by. My friend needed to charge his phone to enter his house (his home gates and door are unlocked via phone) so he went over to my gf's to do that. However when they got there my girlfriend became very flirtatious with him asking him random questions on how to tell if someone has a crush on you, was trying to get his attention. My friend also revealed on how she told him she'd like to be more free etc. She also had suspicions that my friend liked her and she wanted to find out which makes it even more disturbing for me. However, my friend stayed loyal and told me everything after a few days. I was obviously very disturbed by this and confronted her immediately with the intention of breaking up. But she seemed very apologetic and cried intensely saying how it was a mistake for her to act this way and how she still loved me and that can't be changed but she did say that she felt a bit charmed by him at the moment. I actually agree that she always cared for me, loved me and these emotions were never faked which impels me to forgive her. But the boundaries set previously were very clear and I always considered trust and loyalty the foundation of a healthy relationship which she obviously knew. I think this wouldn't be such a big thing if she confessed it to me rather than my friend telling me this. I also previously thought that the two of them were spending a lot of their time together and had a gut saying there are some feelings involved but she brushed it off saying how it's not a big deal and she wouldn't do anything to hurt me. I feel like if I did forgive her I couldn't trust her again and I would always be wondering when would she cross the line again which sounds like a big burden to me.
Anyways, has someone been in a similar situation or can give some advice on what to do here?
TL:DR Gf became flirtatious with my friend when they were alone and never said a word about it to me. My friend later revealed what happened. Should we break up?