I don't know the answer, but there is no harm speculating:
- She has 6 years to complete the 5 years in HPSA.
- If you file I485 concurrent with a NIW, you get a 'request for evidence' after 2 years or so where you have to proove that you are actually working in the sponsored practice location. If I remember correctly, this RFE is a 3 page document. The first page has to be sent back with the evidence of full-time work/employment. The second and third page have to be sent in after 5 years are complete.
I don't know how it works if you don't file concurrently (e.g. due to retrogression, or because you want to use the NIW-I140 for consular processing at the end of 5 years). I don't think USCIS actually sends you an RFE to document your full-time employment, I am under the impression that the RFE is tied to the pending I485.
So, if that is the case, it would be possible to do what you suggested. Use the NIW I140 as a tool to extend her H1b until YOUR GC comes through without actually providing the service the NIW was approved for.
(Here is the problem with this kind of 'creative' use of the NIW: A girl in NY got herself a NIW approved in 1998 for a practice in the styx BEFORE she started residency. She then did her radiology residency on EAD and moved to practice in Florida. She never actually provided primary care in the HPSA location she was sponsored for. When USCIS found out about it, they tore her a new one and essentially invalidated every status she ever had. You can be 'creative' in using NIW petitions and the like, but you have to be careful that you don't wake a sleeping dog there. If they think you consider them to be stupid, they can get really nasty.)