Usually all monies of the deceased are frozen for a period of time, then Probate takes place. This means that all monies owing (bills, loans, etc.) are first taken off the Estate. In the case of co-signing the banks should be able to give you more information. Either the person that the deceased co-signed for continues to pay the Estate money owing, or if they have stopped paying on the loan then the loan is a debt and will be taken out of the deceased Estate.
No one should ever co-sign (not even to a relative or close family) because the co-signer has total responsibility for paying that loan off.
Good luck Marcy