Can We file a case against the mother instead of her Son?

Good day Attorney, Could you please give me some advice of what to do for the brother of my Sister-in -law who borrewed the land tittle of my father to use as bond to work as a collector in a financing company. He was terminated because of using company's collection money amounting to 58 thousand pesos. He signed the termination letter with a promisorry note to pay the discrepancies in full payment Last November 25, 2005 but he did not pay.We've been trying to pressure his mother to settle the said amount because they connot give the information of the where about of her son. It's my parents who settled his liabilities but we let his mother sign a contract with us indicating the months,amount and the source of the payment to be made but nothing happened. We've been trying to find his locaton, and contact numbers but he was not answering the phone. we believed they were hiding the informations to locate her son because even in his Facebook account, he has been communicating with her sister in abroad and we've just heard that he was working somewhere in Manila.I already filed a complain againt his mother for not complying her promissory note with us in Barangay Captain.But the same scenario, She said that it's not her debt, I cannot cannot give another promises because he cannot locate and contact her son.

We are confused now if the mother who signed a contract with us, or her son that they dont know his where about we need to complain to the legal. We don't know what "complain" to fie against them just to give back the money.

None of us here in my family has experienced to file complains. Please Help us!I hope to hear you soon Attorney.

Thank you so much!