He promised to marry her, then he became religious and mended his ways, and realised that she is not suitable for him

There is a man who was not committed to religion and he used to look at his female neighbour, out of negligence on his part, thinking that there was nothing wrong with it, due to the state of the society in which he lives. This continued for a while, without him speaking directly to her, but he was attracted to her and she to him. When Allaah guided him, he realised that what he had been doing was sinful and wrong, so he decided to end this relationship. He informed the girl about this without speaking to her directly, and promised to marry her after she finished her studies. With the passage of time, this person became more religious and realised that this girl would not be suitable as a wife for him according to the standards of sharee’ah, because she was brought up by a bad mother, so he decided not to marry her. But now he is confused as to what to do. Does he have the right not to marry her and break his promise to her? Is there any sin on him in that case? Otherwise what is the solution? 
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.