Every state has adverse possession laws, and while most take years, there are a few where it is only a very few, However, one of the requirements everywhere is that use of the property be against the will of the titled owner. When she showed him the property line and told him that they could use it if they maintained it, she gave permission, and that prevents a successful claim of adverse posssession; of course, it would be much better to send a letter giving permission. And definitely have it be one that they must sign for.