Friend disaster

Ladies I need help desperately. My friend was dating a guy about a year ago when he suddenly fell off the face of the earth, fast forward a few weeks and we learn that he's set to go to prison for dating a 12 year old, him being 20, a few weeks after that and she starts talking to him again, that already had me upset enough, now she's gone to visit him and just told me that she's dating him!!! What am I supposed to do? This is the dumbest move she's ever made but I don't want to lose her as a friend by telling her how it is. Sure she should understand if she's a good friend but I hate fighting with anyone.