It's highly likely that the stem cells present in the donated liver were able to produce immune responses that were compatible with the donor liver AND the patient's own immune system. As a result, the "factors" that would indicate a foreign body or a natural body all pointed to "natural" in this girl's body.
As a sufferer from an auto-immune disease myself (Type I diabetic for nearly 25+ years now. My own immune system has made my life a living hell), this news is quite intriguing.
I work in the pharmaceutical industry and it is AMAZING how scientists develop new compounds to treat various conditions. At the same time, it's sad to see how finances play such a large role in what is researched and what is not. I almost am battling myself since I want to see research into the cure for Diabetes, yet I know that the treatment of this ---smurfing--- disease creates so many jobs and money in this world.