Why did the sheikh respond by advising the young man whose fiancée was mixing with young men to leave her and search for another, while he instructed the young man who had committed an immoral act to repent?

It is permissible to marry a girl who previously lived an undisciplined life if she has repented and mended her ways. However, if the suitor doubts her sincerity, it is better for him not to marry her, and the presence of virginity does not preclude doubt. There are cases where a man becomes convinced of a woman's righteousness and marries her seeking reward from Allah. Whoever is asked for advice should advise with what they deem correct and appropriate for the situation. For anyone who has fallen into forbidden acts, it is advised to repent, return to Allah, seek forgiveness, and conceal their past, whether male or female.