here is some info to treat arthritis:

First off, has your friend really had a biopsy done on the skin to positively confirm the diagosis? The reason I question is that I was diagnosed with the same thing years ago. I then sought a second opinion from a dermatologist who did a biopsy. It wasn’t psoriatic arthritis at all. It was Lichen Planus.
If but, it is Psoriatric Arthritis, then I would highly reccomend any a Rheumatologist, or a Homeopath/Naturopath. Personally, I prefer the Homeopathic approach. There are no man-made chemicals involved, which our bodies are not designed to assimilate. Introducing an artifical chemical to the human body regularly times makes an alternate imbalance somewhere else, with its own set of problems.
Your friend is very lucky to have someone like you in their life.

I do not judge there is a cure because it is caused by immune system problems. But, an arthritis specialist can prescribe medication that will help a lot. See a specialist in immune system and arthritis.