I Cant Trust Her Anymore [advice please]

Here's the story. There is this girl that I was sorta kinda friends with. She asked me for advice. She wanted to know if she should date her friends ex, they just broke up a day before, or not. I told her its not the best thing to do right now. If he really liked you then he would wait a while, and no guy is worth loosing friends over. She said yeah, that she isn't going to date him. About two hours later she text me saying she's going out with him. I was so mad. See this isn't the first time she's done it. Its not the second or third or fourth either. Never the less she's done it to me twice! I told her that I couldn't a friends with her anymore. If this guy doesn't work out how do I know she won't go after my boyfriend. I DON'T! So yeah I told her that we can't be friends. It feels kinda childish, but did I make the right choice?