If they are comfortable to use a tablet, they should have little problems using any app. If ease of buying is in the consideration too, the kindle app and ecosystem should be the best. Otherwise if epubs are preferred I can recommend mantano reader and fbreader. Both have an easy to use library and simple menus.
If they don't own a device yet, maybe a dedicated e-reader would be best. They do just one thing and they do it in a very easy way. Probably there the kindle is the most easy to use overall. If they don't have wifi and enough spare money, the 3g model would allow them to buy books without wifi or a pc.
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