The short answer is; you can't. Unfortunately, the harder you push a child to not get involved with someone, the stronger they feel about proving you wrong. I have found that the more supportive yet realistic you are about these relationships, the faster they go down in flames. Affirming statements in support of the relationship dynamic make people think about what they want. For instance when a young girl finds a man with a weak resolve (usually a drunk), the affirming statement is "I think he's perfect for you because you can maintain control in the relationship and that is so important to you". Things like that make people consider why they are with that person. First you have to figure out why she is with that person. Is it attention, weakness? Does she always have to be in a relationship? Gear your comments toward thoughtfully pointing out how her bad relationship dynamic is fulfilling her needs. Let her decide to end it.