Sure, I suppose she might have been able to test into some grad level classes, but I'm not aware of any way to actually enter a grad level program without a Bachelor's degree. I may be wrong.
Yeah, I think you're right. It would be believable enough for her to be taking some grad classes in a particular area where she could demonstrate a lot of home-learned background, but it doesn't seem like any graduate program (particularly not a highly ranked one like Northwestern, about top 20 nationwide for math) would accept someone as a full-time student without a degree.