The only way I think a pillow case would do harm is if the person tied the pillow case so close to the snake that it suffocated it. If using a pillow case (or any type of bag) you should allow room for the snake to move which would help airflow get in and out.
Or maybe the person tied part of the snake with the knot when they were closing the pillow case???
Anyway, good look with the adoptions.