A friend of mine wants to marry a Shia girl. I have told him that it is not permissible to do so in Islam but it is very difficult to make him understand. Should I keep telling him till he realizes that what he is doing is not right or should I just keep quite now since I have told him enough times. What will happen to him if he marries her? Will he too become a kaafir?
It appears as though your friend has made up his mind to marry the girl in reference and no amount of advice will dissuade him from his intention. As an alternative, you should advise him to be firm on Deen according to the belies of the Ahlus Sunnah Wal Jamaa.
We cannot comment on the Imaan of the girl as Shi?as are of different sects. Some have Kufr beliefs. If the girl has Kufr beliefs, he cannot marry her. His Nikah will be invalid and he will be leading a life of Zina with her. He should first endervour to correct her beliefs and bring her in line with the Ahlus Sunnah Wal Jamaa teachings and then marry her. If she does not have Kufr beliefs, Nikah to her will be valid but in this case too, he should endervour to correct her beliefs completely.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai