My suggestion would be to take several companies' placement tests. Also, since she is an adult, you should be able to have several discussions with her about what she likes/dislikes, how she learns best, etc. Would she prefer a workbook (ACE perhaps?), textbooks, real (living books)? Does she need some basic literacy training?
For general knowledge, history, even science... think about videos or books on tape. Shows like NOVA can be an education in themselves for an adult, even if you don't understand it all.
I have an acquaintance who helped her sis in law a few years ago. Mainly, she helped her pick books from the library (her reading skills were good), they watched and discussed documentaries (with popcorn!), and she worked through a Saxon text book... 87, if I remember correctly. Unfortunately, I lost tract of my friend, so I don't know the end of the story.
Emphasize LEARNING the material and don't test over the material. Get some test practice books and use that to prepare for college entrance exams.