This man is doing the arm pillow completely wrong. The position is correct, behind the neck so less weight is on it, however, the optimal way to lay like this is on their sides either facing each other or spooning. This lets the shoulder lift the neck a bit and provide a gap to lessen the weight on the arm, this also has the added benefit of allowing him to wrap an arm around her and hug her. Alternatively she could also shift position and lay her head on his chest which could take the weight better. Source: Spent like 20 minutes trying to find an optimal cuddling position with my girlfriend awhile ago.