What would you do if your brother or sister was dating an Arab/Moslem/Black?
No different than dating any other kind of non-Jew. It's a terrible tragedy to intermarry.
Actually, I'd be more vocal about it if was an Arab/Muslim to be honest. Because I'd have more ammunition that way. I would protest his not only marrying out but marrying the enemy of his own people....
God forbid this should ever happen.