I love Moon+ Pro Reader but it's not really for a novice I don't think. Mantano or Aldiko
Mantano has collections and is very easy to use. If I was going to get my sister into reading epub it's the app I would get her.
However for a beginner I have to say the Kindle for Android is an awesome app. It's probably would be my second choice. You could email her books directly to her app once you set it up on Amazon. It also has collections and books can be sideloaded . It's not epub though but you can't get a more simple reading app.
Moon+ I love because I no longer have to reconvert my epub books I can just pull the book from Calibre folder on Dropbox. I do have Calibre Companion installed and I really like it but it's too much work for me. I never remember to add the books and then sync all the time. So right now I use my Dropbox app and just grab a book and import it into Moon. Moon will override most of the styles so I no longer in Calibre need to add a indentation, a blank line after every paragraph or remove fonts because it will auto do that for me
I now only reconvert if the table of contents is missing as that is needed for Time Left To Read feature.