Sorry to hear about your friend. When I studied with Roy Stevens, he told of a student of his who had had some nerve damage. One of Roy's exercises helped the man overcome this and regain strength.
He called it the pencil exercise.
Place a pencil between the teeth and slowly raise it to a horizontal position. This does a number of things. It gets the student to get used to holding an open teeth aperture, and it gets the student to bring his jaw out to support the mouthpiece more efficiently. Do not bite down in the pencil hard but just lightly steady it.
Next, without pulling back the corners, GENTLY bring the center of the lips toward you until they touch the pencil. Now GENTLY begin to exhale a Silent breath - not "blow" as this will cause tension in the lip muscles which is not associated with resisting the air. Do not allow the corners to move. Because the pencil is causing a teeth opening, the muscles in the lips and cheeks will have to slightly tighten to just stay still. That is the goal. As more and more air is exhaled, the muscles will tense that much more just to not move. Your friend should exhale ever so slightly. His mouth will shift because of the lack of muscle control in that top lip, but it will get stimulated.
The brain is a marvelous thing and hopefully it will figure out a way to reactivate these nerves and muscles. And his whole embouchure and cheeks will get a workout. I hope and pray that this will be helpful and that he will have a complete recovery.