ANSWER:

He must not expose his sins and can be diplomatic by saying something like he knows of her from someone who doesn’t want to be disclosed 

QUESTION: If someone broke contact with a girl because it was haram and now he wants to ask her hand. What is he supposed to say when his parents or the wali of the girl asks him from where he knows this girl? Because it is haram to share old sins and says that he talked to her and that he knows her.