Different states have different rules, but they usually require:
The owner probably could have prevented the AP if they noticed it and protested before the adverse possession time lapsed.
As mentioned elsewhere in this thread, granting a license (permission) usually prevents adverse possession as well.
Folks unaware of how adverse possession works in their area should also research implied easements -- they have some similarity to AP and are far more common.
In Delaware it doesn’t matter if you were paying the taxes or there was questionable adverse possession, all it takes is a judge making a case for the claimant and you’ve lost the land to a scumbag who never paid for the land or had a deed to it. Been there.
It costs a fortune to fight it in court and appeal it so you lose money and your land is still stolen.
I have a question: If he had gone to a lawyer first, when this started recently, could anything have been done?
Yet the entire process enables the government to take form the landowner without compensation... unlawful taking is theft.