I was just looking to see what e-readers may have text to speech options when I came across your post. I really can't help much with the e-reader, but I have some other suggestions for resources that may help if you aren't already aware of them.
The first is the Library of Congress, National Library Service for the Blind and Physically Handicapped. Your Granddaughter (if she doesn't have one already) could get a digital reading machine from them and you order books on your computer and download them immediately. (202) 707-5100 is the number for the Library. Something else I recommend doing is calling State Rehabilitation Services for whichever state your Granddaughter lives in and they should be able to get her set up with a computer for her homework etc.
Another excellent resource is the National Federation of the Blind (NFB). They have offices in every state and large city. They have a youth division as well as a division for Parents of Blind Children division. Their resources are unprecedented. And while not as familiar with them, another group is The American Council for the Blind. And while I believe they are a good group, I think the NFB has more members and resources.
I'm sorry, I know you were asking for help with an e-reader and I couldn't really answer that one, I do hope that your Granddaughter and her parents reach out to any of the above as they can really help get your Granddaughter some of the tools and technology she will need to be successful and they also have knowledge of financial resources to help out. Good luck to all of you.