ANSWER:

The lender must pay the zakat over his money which he had willingly given to someone else. 

The borrower must also pay zakat if the money is in his possession and a whole year passed on it.

QUESTION: If the borrower is wealthy and is using the saved money of the lender and also wants to pay zakat on it with his own money, will this be against the rules of zakat as the lender (owner) is the one obliged to pay?