Anyway, don't return it yet 13000.
First of all, how much did he borrow? This part of the money is divided between two people, only one person is half. Then figure out how much this half is. If it is less than 13000, don't panic. Just give him back the difference.
If it exceeds 13000, it will be troublesome, which means you have to ask him for the insufficient part. This is usually difficult, so prepare for the worst-sue him if you can't get it. You may need evidence of their cohabitation and loan details. You must find a lawyer for what you need. This is not clear here. (If it is possible to get to this point, you should pay attention. Don't leave evidence to admit that you owe the other party 13000 or receive 26000 bride price, which will reduce the difficulty of the other party's proof. )