disoriented by the whole thing that she is almost downright rude.  She keeps insisting she is from New Rochelle and saying that she already has a best friend.  Soon, however, Rachel’s enthusiasm wins Hannah over and the longer she stays in the past the harder she finds it to keep the barrier between her Chaya self and her Hannah self.  She becomes more and more Chaya.

Chaya entertains Rachel with plots from movies and books, and the girls become friends for real in the days leading up to the trouble.  Hannah almost forgets the time period she has been transported back to, and what is about to happen.  There is not much she can do, of course, and no one believes her.  She has availed them of so much fiction that anything she says is just more fantasy.  Stories of Nazis and concentration camps, and the real danger they are all in, are bound to fall on deaf ears as just the plot to another movie.

In this interesting piece of historical fiction and time travel, Yolen crafts a tale of a girl who doesn't quite fit in with her own world.  When she somehow finds herself in the past, she understands her family history in a way that she never could, and perhaps never should have had to.

Rachel is excited because Chaya is from Lublin, a big city.

Chaya was Hannah’s Aunt Eva’s friend, and Hannah was named after her for her Hebrew name.  When Hannah first experiences her vision or dream, she hears someone calling her Chaya and remembers that she was named after someone.

Chaya. But that's my Hebrew name, Hannah thought. The one I was given to honor Aunt Eva's dead friend. Weird. She wondered how the woman knew that name, then laughed under her breath at her own foolishness. (Ch. 4) 

In the dream or time travel, Hannah is Chaya.  She has traveled back to World War II, and taken her Aunt Eva’s friend’s place.  Hannah does not know what is going on, but since she is there she just becomes Chaya. 

Gitl tells Hannah that her clothes had to be burned due to illness.  She gives her own of her old dresses to wear, and it fits perfectly.  When Hannah looks in a mirror, she sees Chaya.  It is unsettling, but again she goes with it.  

Outside near the barn, Hannah meets another girl named Rachel.  Rachel calls her “Lublin Chaya.”  She is very excited to meet someone from Lublin.  She is also very chatty. 

"You see, we have all been waiting to meet you," the startled-looking girl explained, the breathiness in her voice more pronounced. "Ever since we heard you were coming. Imagine, someone from Lublin living in our shtetl. (Ch. 6) 

When Hannah meets Rachel, she is happy because “the dream—or whatever it was—would be more interesting with girls her own age in it.”  Rachel tells Hannah/Chaya that they are going to be best friends.  Hannah tells her she already has a best friend named Rosemary, and Rachel is shocked that Hannah’s best friend is Catholic.

The Devil's Arithmetic by Jane Yolen employs a unique kind of flashback experience for the young protagonist, Hannah Stern. She is a twelve-year-old girl who lives with her family in New York and is, shall we say, disinterested in hearing about the things which interest the rest of her family about their Jewish heritage. In fact, Hannah wants nothing to do with their remembrances and reflections on the past, despite the fact that her family--like so many others- was impacted by the atrocities of World War II and the Holocaust.

In a rather surreal moment during a Seder celebration, Hannah goes to the door to see if the prophet Elijah has arrived. When she looks outside, the scenery is nothing like what she should be seeing, and when she looks back inside, she is shocked to see that her house has changed, as well. Hanna soon learns that she has somehow been transported into to Germany in 1942 and her name is now Chaya Abramowicz. She arrived from Lublin two days earlier, following the deaths of her parents. SHe has also been ill with the same sickness that killed them.

On her second day in this vision, Chaya attends Schmuel's wedding with Gitl but feels rather out of place since she does not know anyone. That is when Chaya meets Rachel. Rachel has remarkable green eyes and a welcoming smile. She says:

“So—you are Lublin Chaya.”

Rachel is obviously thrilled to finally meet the girl she has been told about by Gitl, and after joyfully introducing Chaya to her friends, Rachel exclaims that Chaya is going to be her new best friend.  

This pronouncement causes Hannah (as Chaya) to finally commit herself to living in this time and place when she realizes that her life in New York would not be acceptable here.

So, the answer to your question is that Rachel has heard all about Chaya from Lublin and is just happy to finally meet her and wants to kind of adopt her as her new best friend.