Probably someone mentioned this already somewhere, but when Ennis tells Jack about Cassie, he says she's a waitress, "wants to go to Nursing school or something."
In the short story, he says she's a waitress, but he "don't know if he wants to deal with all her problems." (quoting loosely; sorry.)
I think the reference to "nursing school" is certainly a reference to the actress, Linda Cardellini, who plays a nurse on ER.
Sort of saying that she waited tables for a while, then went to nursing school, and became a nurse. Not really logical--the character's name is Samantha on ER--but just a little smile.