I am presuming that the loan is not interest based (riba) as this is a major sin. As for the question, if they have money saved that a whole year passed over it and it is more than 1200 dollars, they have to pay zakat over that.

My sister and her husband had applied for the bank loan to buy the car. They want to know is zakat wajib on them in this situation when they owe to the bank?