is this a dealbreaker?

i have been dating my bf for about 3 months and i’ve known him since high school (im 20). i just recently found out that he and my best friend had sex two months before we started dating. ive talked about how much i liked him for so long and she still got with him. part of me wants to give her the benefit of the doubt and say that we weren’t dating yet so it’s fine and the other part wants to say that it’s super disrespectful, not to mention that they also lied this whole time. now every time i see them i can picture it in my head and i wanna throw up 🤮. is this detrimental to our friendship or to our relationship or should i just get past it and move on. i want to hear your thoughts…