How might things have been different if Rivka had not been Hannah's friend in The Devil's Arithmetic?

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.