First she is not you best friend unless you have no other friends. Engaging in sex with the sex partner of a friend is a serious betrayal of trust. It is something that honest and honorable persons almost always refrain from doing.
As to your boyfriend, it is not unusual for a man to give in to the temptation of experiencing an extra casual sexual encounter, especially if he is confronted with an unexpected and unsought opportunity. Still such conduct demonstrates the he valued the momentary pleasures of a little extra sex more than his relationship with you in the event his actions should be discovered.