Does the repentance of the fornicating young man who promised to repent get accepted, and is it permissible for the girl to marry him?

If a servant has been excessive in sinning, then repents, Allah accepts his repentance and replaces his evil deeds with good ones. This is provided that the repentance is sincere (tawbah (definition) nasuh), involving an immediate cessation of the sin. It is permissible for a girl to marry someone who has repented sincerely from fornication, for "the one who repents from a sin is like one who has no sin," and repentance nullifies what came before it. The young man should not have disclosed his sin, which Allah had concealed for him.