I absolutely love it. Even perhaps, buy a brick/building materials type shareholding, rather than ultimate ownership scenario?
He could have Nic the tree man clear an area in the grounds as a campsite for visiting young people/schools, volunteers and families of those that were there during WWII - maybe even part of their European gap years for some?
Or much like most UK schools do, a fieldtrip that my son once did - came home looking like he hadn't washed for the entire week but enjoyed it immensely. I even had to scourer his legs to remove the built up dirt!
Lots of community colleges would love the opportunity for their building/landscaping/horticulture students to work on such a project - it happens in many historical buildings in the UK - why not France? I would have jumped at doing so when I was studying.
Obviously, firstly Dan would need to install a campsite type bathroom block, initially will have to spend money, but longer term would make absolute sense.