Does he have a deed?
For adverse possession to work, it has to be adverse.
Adverse possession laws vary by state. In Michigan the homeowner has to prove that he gave permission, unless the possessor acknowledges that he did. A dated statement signed by the possessor is best. Proof of delivery of a registered letter sometimes works. The time of possession must be at least 15 years, so the permission has to be renewed within that time, and the clock would start over.