Trichotillomania, as it's called, is generally considered to be a form of OCD - Obsessive Compulsive Disorder.
Others have mentioned to check for untreated Neuropsychiatric or Neurodevelopmental, in particular, disorders, and I agree - it's very important to figure out if there is more to this than meet the eye. AUTISM is known to cause behaviour similar to OCD - "stimming" or "stereotypical behaviour" as it's called - OCD can also occur as a result of the anxiety caused from miscommunication with neurotypicals as well.
I suggest you look into the most recent neuro-imaging and genetic data on OCD - and how NMDA-antagonists and 5ht1d-antagonists have great promise here.
SO... Medication-wise, I'd suggest a combo of Vortioxetine (trintellix, brintellix) and Memantine.
Low-dose Ziprasidone could also be beneficial, as it is the commercially available compound with the highest affinity for 5ht1d - it's second highest antagonistic action is ALSO beneficial - 5ht2a-downregulation is theorized to be one of the mechanisms which SSRI's treat depression with.
Good luck, and hope your GF beats this issue.
PS: I don't suggest the "breaking an addiction" - method... there's no evidence whatsoever, and the effects may not carry over to the real world at all, as some theorize that OCD-behaviour is a coping-mechanism for other issues, possibly caused by the environment, i.e social phobia and stuff like that.
Just because she learns to not prick her skin in a locked room, safe and sound, doesn't mean she'll be able to stop it when something unforeseen happens out there in the real world.