Fiance's Terrible History

Q: A friend of mine is getting married and her fiancé has a terrible history, and there is suspicion that he continues to take part in these haraam activities. Is it my duty to notify her, anonymously of course?

A: It is definitely your Islamic duty to inform her of whatever information you have about the person she intends marrying, especially since she is courting that person as well. This is not gheebat or scandalising, provided your information is correct.