He may have acquired or be able to acquire title to the land on which his shop sits by "adverse possession" (what you call "squatter's rights"). If someone occupies land "openly, notoriously, and continuously" for 10 years--so in way that it's obvious the owners, if they ever looked at their land, that he is occupying it, which means they'd have the opportunity to remove him if they wanted, and he does so without a break for 10 years--and he did not have the owner's explicit permission to be there (since occupying with permission does not let you take the land from the personp[s] giving you permission), you can acquire ownership. The sister received the land 24 years ago; if in that time she never told him that he had her permission to occupy the land, that may let him make a claim. It would be worth it for him to consult with a real estate attorney to explore this in greater depth.