Should I forgive my bestfriend for kissing my boyfriend?

My best-friend kissed my boyfriend at a party that my parents did not allow me to go to. My boyfriend told me about it. He said that she came up to him and kissed him on his lips. When I asked him if he stopped the kiss he said he did right away but then I found out they kind of hung out at the party after the kiss. I broke up with him. we dated for a year. but my bff has been there for me since 4th grade and we are juniors now. I don't trust her anymore but her mom tells me that she is hurt and crying a lot and my mother is asking me to reconsider. I don't know what to do?