Unuhi 3 points
NLS has some different models of players.
Sleep button is perfect, and the adjustable speed is really nice feature too.
Humanaware and a few other blind tech companies have book readers with similar features.
But you can look at a few other simple, cheap options too (as money can be an issue).
A basic portable CD player for those audiobooks she can get from a regular library. Most have short chapters so it won't get that lost even if she falls asleep.
A basic walkman type CD player certainly costs less than a new StreamReader or similar as new.
Or import the audiobooks with itunes to her iphone, ipod, ipad, whatever she uses (again, the nice thing: they don't have to be the newest model of everything just fod books), and play them a chapter at a time.