Best friend and boyfriend

I’m a 19-year-old female and I’m currently dating a 21-year-old guy. We used to date in high school, but we broke up and lost touch for three years. Recently, we managed to reconnect and get back together. He just revealed to me that six months after our breakup, he kissed my best friend, who is 20 now. He mentioned they were both drunk and quickly realized it was a mistake, but it really hurts to know that this happened while we were friends. Now, two and a half years later, I’m just finding out about it, and I’m unsure how to react or if I should confront him about it. What hurts the most is that she chose not to tell me. Should I be concerned about this, or is it something I should let go?