You need to forcefully take matters into your own hands because she is clearly being played, and therefore, under the drug of "love," does not have her best interests in mind. You need to clearly and strongly express your dissatisfaction and disagreement with this relationship to all people involved.
You and her family needs to let him (and his family) know that he is an inferior person, and not welcomed around her, and that it would be in his best interests to avoid her, despite his love or whatever. This should be made in a serious and threatening, but legal, manner.
Her family should also put pressure on her to not be involved with this guy. Depending on the situation ,there are some things that only the family can really do, so you may have to work with them. They need to threaten to disown her and refuse to provide for her financially if this marriage were to occur.