There is no single right answer to this question, as it is a matter of speculation. There are a couple of things which should be considered, though. First of all, if Rivka had not been Hannah's friend, I think it might be argued that Hannah might not have survived her ordeal at the camp. It is Rivka who instructs Hannah in the workings of the camps, and what to do and not do in order to survive. Rivka has been in the camp for a year by the time Hannah arrives. Except for her brother Wolfe, who is "one of the walking dead," the rest of her family has been killed and cremated for one reason or another. Rivka teaches Hannah some hard lessons which will enable her to stay alive. Among these are accepting the fact that she must let people go who are so traumatized by the experience that they have stopped caring, and learning never to ask why, because the answer to that question will be enough to drive her crazy. From Rivka, Hannah discovers little details of behavior that will keep her from drawing attention to herself and getting herself killed. She learns to avoid the Greeks who do not speak Yiddish or German and so are quickly targeted for elimination, and how to organize needed supplies and amenities that will help them survive the brutal conditions of the camp. Rivka herself organizes for Hannah a comparably tolerable job in the kitchen. She has recognized that Hannah is a city girl, and would not long be able to take the grueling outdoor work to which she would have been assigned otherwise.
Most important of all, Rivka instills in Hannah an appreciation for the power of remembering. In the last scene at the camp, Hannah gives her life so that Rivka can live, to carry her memories of how the Jews survived the unspeakable into the future. Had Rivka and Hannah not been friends, Rivka would have died, and what would have happened to Hannah, who is inexplicably from the future, is debatable. As it is, Rivka lives to survive the camp because of Hannah, and, as Aunt Eva, gives emotional support to her traumatized brother Wolfe throughout his life, sharing the traditions and stories of the Jews with her extended family, who, after the war, emigrates to America.