You sure you aren't the protagonist of a TV drama? More seriously, this is a ****** up situation. And you need some new friends, and- much later on- a new boyfriend. I'm sorry that not only did your boyfriend cheat on you, but did so with someone you consider a friend. That your "best friend" not only didn't reject your (ex?)boyfriend's advances and inform you of them, but actively participated in the affair with no intentions to ever inform you is just plain disgusting. I suggest getting tested for STDs and pregnancy, if you and your boyfriend were sexually active at ANY point. Even if you used protection or even multiple forms of contraceptives, get them done to be on the safe side. It may make you feel better to know that it's extremely likely that the two of them will cheat on each other multiple times. Their "relationship" will rapidly degrade once the relationship has lost the "forbidden fruit" aspect or the thrill of performing an immoral action or being discovered and is also no longer in the courting phase.