I don't think you can get "retrospect" NCB for being a named driver for the past 3 years.
What I mean is, if it was a brand new policy with say Direct Line and she was a named driver, then yes, she could gain NCB.
However I doubt very much if you could go and get a policy now and claim that the person has 3yrs NCB.
There would be no proof of this and insurance company's tend not to just "accept your word".