Suspect the psoraisis may turn out to be contact dermatitis. The difficulty is trying to work out what causes is.

Often it is caused by the residue of fabric washing products - washing machines leave them in huge amounts. Does she happen to sleep on that side? If so, it is probably that the combination of warmth and sweat is activating residues left in bedding or nightwear.

She should avoid washing products that contain enzymes at all costs. Best of all is to dispense with them altogether for most washes - most dirt is water soluble anyway. If used, rinse until the water is actually clean looking. Then try tasting it. You will be amazed!

 Hope it helps

steve