I need some ideas for revenge?

So my former best friend stole my best friend's (who also happens to be her best friend too) boyfriend. Then she goes round boasting to her friends about how she's gotten eaten out be the boyfriend, and how shes now gone the furthest then the rest of us have. We're not suppose to know, but I found out, and now I want revenge on behalf of my friend. What should I do? It has to be humiliating/hilarious, but can't reflect badly on me or my friend. Any ideas?