Does a girl who met a young man she knows commits adultery, with the intention of advising him against committing this grave sin, have to repent, especially since her conversation with him had no effect? And does frequently committing sins make the heart hard and lead one to trivialize major sins and find the forbidden easy?

Sins have evil effects on the heart, including belittling them. A woman's conversation with a non-mahram man for a need is not a sin requiring repentance, if it is free from forbidden speech, or a submissive tone, or seclusion. However, it is not wise for a woman to advise a man about his sin, as that could be an opening for Satan, and therefore, such an action should not be repeated in the future.