Then they're no longer Muslim. They're no longer following allah's edict not to be friends with Jews or Christians as they are "the worst of creatures".
UNLESS it's a ruse to lower the Jews' guard down, to allow the foxes to mingle with the chickens. Taqiyya. It's like Catholics not believing that the Pope is the vicar of Christ, no longer praying to cannonized saints and the Virgin Mary - then they'd no longer be Catholics.
But if this is their real sentiment, then they will be killed by other Muslims as "hypocrites". There are commands in the qur'an regarding this. This is how serious this thing goes.