tar, dithranol (anthralin), corticosteroids like desoximetasone (Topicort), fluocinonide, vitamin D3 analogues (for example, calcipotriol), and retinoids may be very effective and safe
or total body treatments such as pills, light treatments, or injections like methotrexate, cyclosporine, fluoruracil and retinoids. Then phototherapy can also be used.
I suggest you to consult her dermatologist and discuss these treatment options with her. There is no specific diet for psoriasis.
I hope it helps. Take care and regards.