I got this response elsewhere:
"You are feeling the 60 Hz induced voltage in your friend's body, brought on by capacitive coupling to the wires in the heating mat. I've felt the same thing many times on the surfaces of appliances and light fixtures. Those of us who live in the electrified parts...
I'm wondering about what's going on in an effect I've noticed with heating pads (as well as electric mattress pads).
Try this experiment: take an ordinary heating pad (one of those flat rectangular things about 1 foot by 1.3 feet with an electric plug), plug it in, and have a friend sit...