That convention is Polish, not German. Her surname is still Rejter but when written in Polish, in context, they would, for the female, use Rejterow. It is not quite like the "son of" concept where the surname actually changes. This is only a Polish grammar issue that applies to females. It is not a change of surname issue.
The j is also a Polish construct which adds the long i sound. The normal German would be Reiter.
Similarly you will see place names in Polish with a variety of endings that reflect the way that place name is used in the sentence. "Gostynin is the location of the event." would probably have a different suffix added to it than when used as, "He lived in Gostynin." I know just enough Polish to recognize when these suffixes change but not enough to know what suffix would be used in what situation.
Germans in this region had a more of a tendency to become Polonized than in other parts of Poland. They would often change the spelling of their names or even change to the Polish equivalent (e.g. Schwartz = Czarnecki and both = Black). Also, most of the Lutheran Churches in this region would conduct their services in Polish rather than German. Significant numbers of Poles in this region were Lutheran.