If it’s possible I would have the current homeowner explain to you and your client how they operate the system. They most likely have a primary heating source that is tied into the wood boiler. There appears to be many different opinions on the best way to hook into your current system. Like others have said, disclaim it but try to learn more about it. Chances are it is now shut down for summer unless they are only heating the hot water/pool, etc with it. Just look for the obvious issues that you can see.