I've operated the Zen Stone, an iRiver T60, a Samsung YP-U2 and a first generation Shuffle in a pocket without looking at the controls (the Samsung and iRiver have a small screen).
The T9 would operate something like the Rio Carbon (which I can also operate "in the dark") -- overkill, yes, but I checked out prices for the factory refurbished units, and the prices were very attractive.
And what about the iRiver Clix2? More than you were considering perhaps, but it's all screen and the click controls might work for your mother. If the original Clix is less expensvie now, that could be an option, too.
Does your mother need any bookmarking features for audio books? Some players apparently do better than others regarding that.
At a higher/more expensive end of the scale, and mostly as an anecdote, my aging mother had a 3rd generation iPod I had gotten her, and she used and enjoyed it a lot. Then she went out and got herself a fifth generation iPod Video, and she still doesn't realize it plays video and would display family photos -- she said it was nice that iPod made the screen larger, because it was getting hard for her to read the screen on the third generation model!
