All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Dear sister in Islam, we would like to shed more light on the points that were not clear enough to you.
Firstly: You should know, that the man in question is a non-Mahram to you as long as you haven't married him. It is forbidden for you to show him your adornment or be soft in speech with him and to stay alone with him.
Secondly: The Fatwa differs according to the context of the question itself and the circumstances of the questioner. Hence, we do not advise a man to marry a woman he does not like, as this could have bad consequences and could lead to early divorce. But, we do advise the man in this situation to deal kindly with his parents while trying to convince them of his own choice and to reject theirs.
Thirdly: As your question emphasized mostly your fear to cause harm to the girl to whom the man is engaged, we told you that this is not your problem because you didn't urge him to leave this girl and, therefore, it is not your concern.
Fourthly: Islam gives an utmost importance to the choice of the husband or the wife, as the good choice is the beginning of the couple's happy life. In fact, the crux of this matter is not in the abundance of knowledge. It is rather in the application of Islam and the good adherence to its duties, morals and conduct as well as to the rejection of all what could lead to committing sin.
Fifthly: Our advice to you is to make sure that this man is a true and good Muslim. Then tell him to solve his problem with his family in a good and wise way. If he succeeds to do so, accept him as your husband. But if he fails to convince his parents, we think that it is better for you not to get married to him and not to encourage him to disobey his own parents as this could be a seed to a bad relation between you and his parents.
Finally: You should not carry out any important decision before performing Istikhaarah prayer.
For more benefit on Istikhaarah prayer, please refer to Fatwa 81434.
Allah knows best.