He promised to marry her, then he became religious and mended his ways, and realised that she is not suitable for him
Please note that there is a young man who was keen to propose to this girl, but when he found out that the person who is asking this question wanted to marry her, he gave up the idea.
This young man should praise Allaah for having helped him and guided him, and diverted him from the ways of haraam and evil. He is not obliged to marry this girl even if he promised her that. It is permissible for a man to cancel a proposal if he finds justification for doing so, such as if he realises that the girl is not suitable for him, so how about if the proposal has not been made? He has to look for a girl who is religiously committed, of good attitude and good upbringing, and he should end his relationship with this girl and any woman who is a stranger or non-mahram to him.
We ask Allaah to guide him and make him steadfast.
And Allaah knows best.