What should a young man do if he proposes to a religious girl who is observant of her prayers and faith, but she wears tight trousers?

If a woman wears trousers under her modest clothes, there is no harm in that. However, if she wears them alone and goes out, it is not permissible, and she should be advised. If she insists on it and her compliance is not hoped for, then it is not advisable to marry her; due to the saying of the Prophet, peace and blessings be upon him: "A woman is married for four things: for her wealth, for her lineage, for her beauty, and for her religion. So, win the one who is religious, and you will be successful." Thus, religion is the foundation for choosing a spouse.