Hi there - I would strongly advise doing JAA - if she returns to the UK with an FAA licence and 300 hours I think it is highly unlikey she will be able to find any work. N registered aircraft in the UK rely on a legal provision that allows US owned entities (companies or trusts I understand) to base N registered aircraft in the UK. As such, they are mainly bizjets. It doesn't mean that all bizjets are on the FAA register, just some. I know little about where they get their crews, but I guess they use people recruited in the US or very high hour types over here with FAA tickets...really not sure but I would try to find out if I were you - the bizjet forum here would be a good place to begin.
The airlines won't be allowed to hire her with an FAA licence to operate G registered equipment.
Have a look at companies like EFT in Florida - they will do JAA training in the US (another twist!) but it needn't cost the the earth.....
Good luck and I hope your pension stays intact :-)