Confused about mortgage

A friend asked me a question about one of their dad's mortgages.

The situation is their 87 yr old father has a mortgage on a property he sold. The problem at the closing the company made the check out to their dad instead of the to the company who held the mortgage. His lawyer told him to cash the check and keep paying the mortgage, my friend told him not to do this, but he listen to his lawyer. The property has been torn down and redeveloped into industrial or commercial property.

I told my friend I did not think the new owner could have developed the property without making sure the property was free and clear. Their dad cashed the check and never heard a thing from the company. Now their dad is in financial straits and is one month behind on the mortgage and does not know what to do. I told them this is a legal mess and I would seek out some help for them. My friend wants her dad to stop paying this mortgage if he can, they think the mortgage company has been paid off and is double dipping because their dad is old and vulnerable.

What do I tell them, I have never heard of anything like this!