Oh yeah, twice that come to mind right off the bat. The first was my high school best friend. She started having sex before I did. Long before. She wasn't as attractive as I was, so she'd become friends with my boyfriends, then have sex with them behind my back (I was a virgin). Once I discovered this, I simply wrote her off. In fact, she ended up marrying an ex-boyfriend of mine. I wonder if they're still married. I highly doubt it.
Another time, I found out my supposed best friend was you-know-whating my boyfriend (this was after I was no longer a virgin). I handled it the way I felt appropriate. I called her into a bedroom at a party, backed her up against the wall with my hands around her throat and told her to stay away from my man.
After I discovered "my man" was the culprit (many times) I left him, but not without leaving my mark. He'd beaten me often. I finally got my pay back. That's a story all in itself. I try to forget it, but I'll always remember, and I've never been physically abused again by a boyfriend.
Sorry to get off topic. Your question brought back memories that will never leave me no matter how hard I try to let them go.