dump her now, been there done that. don't let a fem ruin a friendship.
and if he tags her, you can say what she didn't do that to you she did it to me. that will keep him wondering and make him feel uneasy.
and if ya dont dump her and she cheats on ya with him you will only enhance there relationship till you find out. sneak sex is the best.
How many times ya here this:
Ican't (insert name here) he is my best friend.
she: I won't tell if you dont.
him: I just cant do that to him.
she: he'll never find out.
wakkka wakka dang dang.
no she wont tell boyfriend, she will tell everybody else. and they told to friends and so on and so on.
oh and btw from experience your friend prob talkin **** about u to her
just my life experience.