I think a $1,500 price is below market. I would snap it up at his price, and put in the new control panel. They obviously had a poor support team that led to poor chemical maintenance and would have made his wife's use challenging.
You would be into it for under $2,000. Then again, you can go to the dealer and buy a new one for more than $10,000. A 7 year old spa, used as you explain, would be a good deal. But then of course, you have to get it moved, and then once in place and operating, you're going to love it.