If she remains registered, Ofsted can inspect. They only inspect around 10% a year. Some of us nannies have never been inspected, even though we have been registered since the scheme started.
Is she part-time or full-time? If part-time then when working for a family that does use childcare vouchers, then I think she can give Ofsted their address, so no need for Ofsted to know about you at all.
If full-time, then perhaps she can notify Ofsted that she is not working for anyone using childcare vouchers/tax credits, changing her work address to the same as her home address. Not sure if Ofsted will do that but it's worth a try.
She can de-register. She should contact Ofsted with regard to resigning the registration, not let it lapse due to non payment of renewal fee.
Ofsted have a factsheet titled: Resigning and cancelling registration from the Early Years and Childcare Register Today the Ofsted website seems to be playing up, so can't link to the factsheet.