First off, has your friend actually had a biopsy done on the skin to positively confirm the diagosis? The reason I ask 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 however, it is Psoriatric Arthritis, then I would highly reccomend either 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 often times creates an alternate imbalance somewhere else, with its own set of problems.
Your friend is very lucky to have someone like you in their life.