I have no respect for people like you and the boyfriend. You didn't even say "slept", you said "sleeping". You don't feel bad at all yet you claim you don't want to hurt her? You're ready to ghost her over a guy, you don't really seem like a "best" friend. You're a terrible person, but she deserves to know the truth. Know you have probably lost someone you claimed to be bestfriends with over this. I hope you're happy.