Is the young man sinful for thinking about marrying his former colleague after his current marriage, and is she sinful for becoming religious and withdrawing from his life after learning of his marriage?

A Muslim man is permitted to propose to another woman, as polygyny is permissible. However, deliberately calling one's wife by another woman's name contravenes living together in kindness (al-mu'asharah bi'l-ma'ruf). A wife fleeing her home without her husband's permission is not justifiable, and she should fear Allah. The husband must mend the relationship with kindness, remembering the words of the Prophet, peace and blessings be upon him: "A believing man should not hate a believing woman. If he dislikes one of her characteristics, he will be pleased with another."