It is a complusive/obessive disorder that she has, she is as they say, an addict.
I read about this from a different source. A friend of mine who was an addiction counsellor has told me about this, it isn't uncommon to replace one addiction with another.
When she worked with ex drug users they had a 'contract' and on it was kepping a diary of activity. It became apparent that many were using tanning/excercise as a replacment for drugs, so they were not in recovery as thought.
This woman has just gone straight to tanning for her 'fix'.