My freind, her boy friend, and i!!!! help?

okay so i have a friend an she has been dating this really y dude for lik 9months and of them months i have known her and him for lik 5.so me and her is really cool lik best friends and me n him is really cool too. so we be sneakin him in her house an shyt to stay the nite an some times just so we don get caught i have to sleep in the bed wit him alone while she sleep in the living room so the first nite he was touchin on me and feelin on me n shyt (now remember they are in"love") i jus let it slip even though i should have told him to stop but i didnt. an then it happenened again a couple other nites an then one nite(i feel REALLY bad for this) but we did "it". i don want to tell her even though i should but now she is really comitted to him but he over here tellin me he wanna come see me one day with out her and stuff and i think im startin to really lik him too an i don know what to do cause when she talks about him i just be wantin to telll her how i feel bout him too. help!