Boyfriends girl friend what do you think?how would you feel?

recently my bf an I have had big struggles an been growing apart a lot anyway we got into a fight an he wanted to prove that hes faithful so he told me not to get mad to stay calm but he was gonna tell me something to prove how faithful he is..him an his close friend were at her house an her sisters left it was just them two in her room an she asked to have sex..I was so pissed by this well anyway he says he told her no an showed himself out the thing is hes been keeping this a secret from me up until now just to try an show hes faithful..if I woulda never got mad an thretend to break up with him he woulkdve never even told me this! an then he even came up with excuses..saying she was slightly drunk/tipsy which fueld me even more..other then the fact hes been hiding it he continues to be friends with her! when she clearly disrespected us an our relationship! this hurts me an awful lot then he can be friends with such a person an keep this from me up until now..what is your opinion on this? am I overreacting? how would you feel?