How to get him back before she hurts him?

Okay, so heres the deal, there is this guy I like, and he is now in a relationship with my now ex-friend{we used to be 'best friends'-in her eyes- before}, and I dont want him to get hurt by her, because she never sticks to one guy for more than a month because she gets bored with them.....She told me the day I myself was going to ask him out, that she wanted to date him; knowing fully well what I was doing ask him out, and knowing fully well that I had known him longer and there had been feelings between the guy and me since we first meet; but she said didn't want it to fast or anything, then bam, three days later, their in a relationship.....now how do I get him back before she hurts him? and should I let him know of her previous escapades with guys or no?