In fact, the poor Friederike had a very very unhappy life when she was a little girl.
She had to endure a veritable martyrdom, because she was suffering from a deformation of the spine, and was forced to spend three years in an hospital, far from her family, during this time she was strapped to an horrible machine, and litteraly streched...
But, this treatment was successfull, and she had a normal life after this!
She was courted by the prince Karl of Hessen-Philippstahl, but did'nt want to marry him (she had a very strong character).
When she had 35, she announced that she was in love with Baron Washington, and determined to marry him! It was incredible, this man was 10 years younger and had been professor of her half-brother Elimar.
Friederike's parents were dead at this time, and her half-brother Grand-Duke Peter liked her very much, so he gave his approval (on the contrary of what he did for Elimar's wedding, in this case he never forgave him).
They left Oldenburg for Pöls (near Graz), and Friederike spent a very happy life playing the role of a farmer's wife.
They had two sons, who had no children.
I don't think she came back in Oldenburg, at least, it's not mentioned in the book I have. It seems that she really broke away with her first life.