My friend wants to get revenge, but I don't want the guy's heart broken?

One of my best friends is so fed up with a certain guy dating a different girl every like two to three months. I was too until I rethought it and realized the reason he hurt me. He's currently going out with a girl named Justine, and they are very cute together. However, my friend thinks they are going to break up in a matter of time. She wants to wait until they break up and then go out with him but break his heart. I was originally in on this idea until I realized that I'm too nice and really not like that. Plus, Matt and Justine, or Mattine, are just so cute together <33. So now I don't know whether I should tell Matt about her plan or not. Or should I tell Justine to hold him tight? Oh, and this is all online btw, on MSN and Wetpaint...so what should I do?