Okay...I have a problem with a "friend"

There’s this girl (she WAS a good friend of mine) she still thinks we’re good friends. But she made a mistake and ran off with her psycho boyfriend. Things got ugly with her and her parents. Her boyfriend gave her a cell phone because they weren’t allowed to talk (this is before she turned 18) and when things got ugly, she told them I gave her the phone. I haven’t got onto her about this, cause I’m the type of person that doesn’t like to hurt feelings. Now, a couple months later, as in TODAY, I just found out from a mutual friend, that she’s telling people I, as in ME, that I gave her this stupid little blue diamond ring. HER STUPID PSYCHO BOYFRIEND…no wait, FIANCE, gave her that stupid ring!!! What the hell is her problem!!! She’s used me before, when things weren’t going her way. Now she’s basically doing it again. and another thing is…I DON’T TALK TO HER! She won’t talk to us anymore, cause her FIANCE doesn’t want her to talk to any of her friends!! I’m sooo POed right now, I’ve never EVER been this ticked off!! I’m to the point where I literally want to kill her, or ruin her life in the worst way!!! What can I do? They live right down the street and I really want to go down there and punch her in her face! Why is she doing this? AND ONLY TO ME!!!