The US will not allow her extradition. They will not allow the wife of a security official to be in a foreign prison.
It's a step by the CPS but one which I don't think will achieve much. I doubt Sacoolas will be allowed to leave the US for a long, long time (if ever).
FWIW I suspect she was pulled out of the UK rather than her fleeing of her own accord. The US embassy informed the FO (i.e. told them in no uncertain terms) she would be leaving after the FO asked the US to waive immunity. She left the country on a USAF plane. US embassy keeps quiet about the fact she's no longer in the UK until the FO actually ask the question.
To me, that suggests she had little choice in the matter and it was in the hands of the US embassy and associated officials, although I don't doubt she felt relief.