When is enough really enough when it comes to friendship vs backstabbers?

i have been seeing a guy on and off for about 5 months but its just been friends with benefits. i met him through someone who is supposed to be my best friend but she is always stealing my guys ever since 9th grade and when me and this guy was off for a little while they had a drunken night and now she is pregnant and me and him are secretly screwing around again. its a secret now because she thinks she owns him and i am the "whore". i know i should quit talking to her but the way she is i am afraid she might kill me! she already has 2 kids with my first love i mean why would i stay friends with her for this long? i am just so confused!