Exposing a person's past to their future spouse

I have a cousin who is supposed to get married to a boy who is pious, haaji and just the opposite of my cousin sister. She is not so pious and had a dark past. She committed zina with another guy, tried committing suicide, and now she is about to get married to this boy who knows nothing about the girls past except that she had someone in her life before. Should the marriage take place without hiding such things about the girl? And if I inform the boys family regarding this will I be committing a sin?