I'll second Nizoral. It's the only thing that's ever worked for me. Not psoriasis, but sebbhoreic dermatitis. My father had pretty bad psoriasis and had good luck with coal tar shampoo. Sunlight is also hugely helpful.

I've used hydrocortisone before and it's worked well for rashes, thanks.

suggest to her adding tea tree oil in addition because I found that hydrocortisone by itself didn't really help as much as using both