In what you think of as the Worldly world- known sex offenders must register with the police so that they community knows who they are. There are searchable databases of pedophiles and sex offenders so you know if you are neighbors with one.
This is because of Megans law.
If so called worldly people can protect children with information- why cant JWs do the same.
Now that you have the information, there is no dilemma at all. Report it and tell the engaged woman.
That is the right thing to do.
EDIT: I completely disagree with DATA DOG. Absolutely do not confront the abuser. You put yourself at risk. She does not need to know every detail WHAT ARE YOU THINKING DD!!!!!!