Need to let the gentleman know, this system has been gone a very long time ago. There are no parts, support, or anything else available anywhere. And even if he does have some spare parts, he would have to find that rare person that is familiar with that system. There were not many and most are not around anymore.

So everyone is talking to, saying to replace it, are spot on.

There is a LOT of info missing to know what to recommend (even you know this one Julie). I'm guessing by the $65k-$85k quotes that this is NOT a small house by any means and the installation was very extensive. Probably to big and to many loads for something simple like Lutron Caseta to replace it.
If the $65k is way to much to handle, I might suggest a quote from an electrician to pull it out and flip everything to manual control. Still probably a lot of work and having to pull new lines to places but I would think it would be less than 1/2 or even 1/4 the $65k quote.

And as far as music goes. Next owner's money pit to worry about. Otherwise just have him get a SONOS 1,3,5 and enjoy. Hopefully it has a wireless network that was done at least 1/2 OK.