Wow, IML....
As a person whose family (and several girls I know) has been deeply affected by predators within the congregation and family "friends", I applaud you and implore you to follow through. The police are a good start, but based on my experience even with the law, you may need to find another way to let that poor woman know who she is marrying.........the elders will be of no help, and perhaps would just accuse you of slandering the guy (EVEN THOUGH HE IS GUILTY)
I personally feel these people cannot be completely rehabilitated - at least in the sense that that urge and temptation will always be with him - and he is going to be like an alcoholic in a brewery with that little girl around. What happens the first night he "babysits" the kids.....It makes me sick just thinking about it.
You are doing the right thing - best wishes with your efforts.....