I messed around with my friends boyfriend
What do you mean when you say "messed around"?
By "messed around," I mean we made love
You may have liked him, but that doesn't give you the right to help him cheat on your friend. You should only try being in a relationship with someone who is single, because you may not only lose your friend but a lot of respect that people had for you if they ever find out.
It's not stealing, if I make someone cheat. I don't see why someone shouldn't be with whoever they want.
I think it's best to just cut off this relationship right away. Having sex with someone else's boyfriend is still cheating. How would his girlfriend feel if she found out from the both of you? It would be better for you to find someone else to be with that is not in a relationship.