How do I tell my close friend not to date a guy?

I was into my one friend and she rejected me for this other guy. No problem, I'll still be her friend. But I've talked to this guy before and I know he's not someone willing to settle. Today, at a poker night he was here and he revealed to me on the drive home he's using her just to bang. He's a really good looking guy so he just does without needing to think.

I know she's someone who wants a stable relationship. Should I tell her this guy is just using her. I'm not doing this so see if I have a chance since I already told her I don't want to be a backup. But as a friend, is this the right decision?
How do I tell my close friend not to date a guy?
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