I think in case she has dual citizenship, she still has to pay into German social security. These payments are mandatory and contributions are accrued over a lifetime.
I think The minimum fee is about 150 Euros per month and family (even as an "inactive" citizen).
Maybe she can refuse to pay ("Hey, I've moved back to the UK / never moved to Germany"), but this also means almost? no pension payments later, and she can expect trouble when paying invoices for any German doctor or hospital.
Of course she can live off her wealth (gained in the UK) as a Pensioner in Germany but I think when she never contributed to German Social Security she cannot expect much. In this respect, Bookkeeping by German authorities is very accurate, over decades, to the penny.