While driving or while at home, Knitting, etc.
I does sound like an overkill to buy Kindle for this purpose but yes, in principle it can be done. The Kindle audiobook player and the speakers are decent.
However, an important restriction may be the space: ~3.2 GB of usable space may sound sufficient but audiobooks tend to be large and there is no way to get more space on the Kindle. Devoted music players or even mobile phones are, or at least can be, of tens of GB.
While it is not important while driving or knitting, it may still be good to know that you cannot use your Kindle in any other way while listening to the audiobook - a restriction that I personally find very limiting and unnecessary, as playing sound in the background certainly is possible - as the built-in MP3 player shows. A pity as being able to list through the book while listening to it may have been a cool feature.
Add the dummy on-device-only collections organization to it.
So in summary, it can be done but I would not do it. If your mom wants an audiobook player then getting one for her may be cheaper and more functional solution, unless you are confident that the mentioned restrictions are not a problem and you are confident that the Kindle would eventually get used for reading books also - in that case it may be good choice.