You did a dumb.
You knew she had a boyfriend and you did it anyway, so I don't really understand why you're trying to shrug off the responsibility. You could have held her at arm's length and said, hang on a minute you're about to cheat on your boyfriend; we shouldn't do this. Instead you ignored it, even though you knew it would leave you in this situation.
Just because she's the one in the relationship doesn't make you innocent, you might be the single one but you've helped facilitate all the heartbreak that will most likely ensue.
"Bit" of guilt? Have some compassion. You didn't aimlessly wander into this as a bystander, you're an active and willing participant in infidelity so I suggest you figure out how you feel about that and either cut it off or continue to make it worse. My personal suggestion is you cut her off and learn from your mistake, but that's down to you. Is it likely she would have cheated with someone else had she the opportunity? Or is it because she likes you particularly? Regardless, you're going to suffer for it now because if he finds out you're in trouble and you'll lose them both as friends.
TL;TR
Distance yourself from her and stop sleeping with people in relationships, it just gets you in a mess.