What proof (AS IN EVIDENCE-BASED MEDICINE) exists that your friend has a mental disorder? Was she evaluated by a professional?
How can a non-psychiatrist diagnose a derm patient as having a psychiatric disorder? For example, a podiatrist shouldn't be telling a patient they have dental caries.
How can a mycologist, one who deals with molds in structures--not the human body--label someone as delusional? My NYU ear specialist told me that top rated, world-class NYU doesn't have the resources/access to resources to properly diagnose a fungi infection (esp. rare fungi) in my ear. Labs are limited in what they can identify by computer as the pathogen must be in their library. Only an experienced pathologist can hopefully identify what is not in the computer.
Remember that parasites move. It is almost impossible to know where a microscopic parasite might be residing in layers of human skin.
One dermatologist told me that the classic education for a condition that cannot produce test-derived positive results would, by default, be presumed to be delusions. In essence, doctors treat signs, not symptoms. This is probably to defend themselves against lawsuits.
Modern medicine is practiced by the book. If it's not in the book, it's not treated.
Think about it. Delusional parasitosis is a growing diagnosis because the way to bill for services where there is no evidence is to code for delusions. Delusional parasitosis is predominant among middle aged and older women, a group that has traditionally been easy to tag as hysterical and hypochondriac.