[ Offer her a place to hide when it goes wrong. ]
About all you can do in this case.
Some people will never learn until they get a few hard knocks from life and even then they cannot.
The pastor at the church she attends must counsel with her that she is choosing that man over the Lord as She is commanded not to marry an unbeliever . Who would perform marriage ceremony? It would have to be a Muslim ceremony.
Her Christian friends should start to separate from her iafter her friends and pastor has counseled her that she is sinning against God. This is instructed in scripture as it may help her to come back to her senses. Poor parents having to watch this happening. Of course God can do anything so asking God to save her out of the situation is #1.