How can I make it stop ?

Last week, I went out with 2 friends in Paris, they told me not to bring that one guy, at some point I ask them why, one of the two told me that 2 months ago during a party, he make her drink to have sex with her, she woke up with him on top of her. The other one told me that apparently he did that to other women too, he make girls drink till they don't say no anymore. And I know that guy, I know he's always around my ex, even when we were together as soon as she posted that we were in a club he appeared from nowhere he somehow followed her everywhere. So I told her to be careful without bringing any names because the friend who got abused doesn't wan't problems, she doesn't her story to be known. And.. at first my ex told me that it happen to her too so we talked about it.. I know how she describes him, that little guy who don't really have his way with women, so I asked her, and we were talking about the same dude... But she doesn't want her story to be known either, she told me "it wasn't really honest of him, it was bad he shouldn't have done it, it was a mistake, sadly we can't prove what he did, but I don't remember anything so it's ok" I know that guy, I know what he did, I know what he do, and I just have to deal with it ? Why would he stop, it work and nobody say anything about it. It pain me it haunt me, I can't say or do anything. My friend told me to avoid the places he go and move on, but why should we be the ones avoiding him when he's the one abusing women ? I tried to talk to them, I told them they had all the rights to drink they also had the right to be safe. I told them that they're not alone, other women had the same experience and like them they feel like alone no one will listen, but together they'll have weight against him.. But nothing, my ex litteraly told me that she doesn't care about the futurs ones she just doesn't want to be involve in anything. So what.. Am I suppose to respect their choices and to bad for the next one ? I don't want to push them or put them into troubles they don't want.. so what ?