Legal Question in Real Estate Law in Maryland

Thankfully for your parents, the law requires that such claims (called adverse possession claims) be "hostile" or "adverse" for a period of 20 years.

This means that if an owner gives someone permission to live at their home, that person should not be able to claim any ownership by adverse possession. While there should be no ownership rights on the fact scenario you raise, there may arguably be a implied landlord/tenant relationship so it may be prudent to give a formal written 30 day eviction notice.