Mortgage is tax deductible if you are itemizing, which most people in the US do not. He's almost certainly not going to be itemizing if he doesn't have an income any more.
Additionally, you probably can do better investing, but we don't know when he took out his mortgage. I got a mortgage in 2000 and the rates were much higher. A guaranteed 6.5% return on my investment? Not too bad. Refinancing can be expensive, and depending on what remains on the mortgage it may not be worth it.
So really, it's almost impossible to speculate on whether paying of the mortgage was a good idea until we know more of the details. I admit I would be inclined to pay it off just for the certainty and the immediate lowering of expenses. Investing can take time to return, possibly more time than he has.
Additionally, you probably can do better investing, but we don't know when he took out his mortgage. I got a mortgage in 2000 and the rates were much higher. A guaranteed 6.5% return on my investment? Not too bad. Refinancing can be expensive, and depending on what remains on the mortgage it may not be worth it.
So really, it's almost impossible to speculate on whether paying of the mortgage was a good idea until we know more of the details. I admit I would be inclined to pay it off just for the certainty and the immediate lowering of expenses. Investing can take time to return, possibly more time than he has.