Good idea Hden. I am not saying that it is enough for her to take vitamin D and then she will be cured of psoriasis.
But if she is deficient then correcting her deficiency will definitely have an impact on the inflammation.
Vitamin D is not a magic bullet for psoriasis, it's just one important factor in the management of psoriasis. There are other elements too like diet, UV light etc.
However vitamin D is in a class of its own regarding supplements because we know it has a powerful modulatory effect on the immune system. Perhaps in the future we will know how to harness its power better.