Can you be proactive about it? When she gets involved with a new guy and starts gushing about him, take the opportunity to set her straight BEFORE she gets to the point of whining on your shoulder. "Friend, I'm glad you like newguy. It's great that it's going so well for you! I'm happy for you, and I want to continue to stay happy for you. So do yourself a favor, and don't sabotage this relationship. You always ask me what to do, how DF and I are so happy, and I tell you, and then you do the opposite and end up breaking up with the guy. Then you cry on my shoulder, ask me what to do, and it starts all over again. It's a cycle, and it doesn't seem that you want to break it, but I do. So here's the thing: I'm not doing that anymore. If you want something to change, then change it! Don't just destroy it and then complain to me. It doesn't do anyone any good. So this time, if you do any of those things that I've told you before not to do, like keep secrets from each other, then I'm not going to listen to you vent when your relationship goes south. If you want my advice, then either use it for once or quit asking for it."
Harsh? Maybe. But sometimes harsh things need to be said. And then you've given her a warning of sorts for what's to come.