I would think it's more the other way round - that is, Christina is a foil to Nora - since Nora is the main character and it's her development that the play is about. However, it's certain that Ibsen deliberately puts the two women together to highlight certain issues.
It's when Christina comes to see her old school friend that it really becomes clear to the audience what is meant by the play's title: That Nora is living in a "doll's house," a specially created, protected environment far from the real world. This is emphasised by what we learn of Christina's life; hard-working, adult and responsible. By contrast Nora is not even able to look after her own children, let alone earn her living. This meeting is important in Nora's re-examination of her life.
There is an essay extract here that migt give some more pointers.
It's when Christina comes to see her old school friend that it really becomes clear to the audience what is meant by the play's title: That Nora is living in a "doll's house," a specially created, protected environment far from the real world. This is emphasised by what we learn of Christina's life; hard-working, adult and responsible. By contrast Nora is not even able to look after her own children, let alone earn her living. This meeting is important in Nora's re-examination of her life.
There is an essay extract here that migt give some more pointers.