Tell Grandma that you researched this on the Society CD ROM and she's right, demons not only inhabit homes and frighten people by moving clothes around, but some people have even got demons by talking to someone on the phone who lives in a demonised home.
Tell her that letters from one person to another can be contaminated with demons, and the best way to protect herself is to break off all contact until he gets rid of them.
Let them start shunning one another for their stupidity and gullability.