Debt Collector requesting amount from relatives

Hi Sir, My wife's brother has defaulted his credit card amount and gone out of station nearly six to eight months back. He has no contact with me or my wife for the same time since we had a family fight and he has to settle nearly 50k to me. A credit card collection man has called me before and said my name has been given as reference and he came to my home too asking for money. I said i will tell him (my wife's brother) abt the same and informed him abt the same and given the same number to the collection man. He too called him and two times i think the minimum amount paid and was smooth. but suddenly the collection man called two days back and saying he is not picking the phone, you are responsible you have to pay the amount. but now when I called my wife's brother he is not picking the phone. I said the same to the collection man but he is demanding your name has been given as reference so if he does not pay you have to pay the remaining amount. What I need to do, pls advise. My wife and my family are a little worried about the situation.