I have a question: If he had gone to a lawyer first, when this started recently, could anything have been done?
Not sure, I didn’t read the story. (I was just commenting on adverse possession in general.)
Generally, if the local adverse requirements are met (including the duration requirement), the adverse possessor acquires vested property rights in possessed the land. It takes 20 years in Delaware, which is a long time as far as these things go.