If he didnt pay for the consecutive 3months, the bank will automatically file a case against him at the police station and he will be stopped at the immigration. He should pay before going out of UAE. Otherwise once caught, he has to pay first the bank. If he has no money, he will be jailed for 23 days for the credit or loan amount of AED 40,000 & below. If he has credit with different bank, he has to serve the terms separately.