Look at an impedance matching Niles box. They come in pairs up to (10?) pairs, which gives you up to (20?) speakers.
To drive this, you simply need sufficient power from any receiver or amp. The box's purpose is to prevent a safe load to the receiver no matter how many speakers are connected.
Note that some of the Niles boxes get quite pricey, and the one I saw that seemed to best fit her needs runs about $700. It's probably not trivial to impedance match that many speaker pairs.
As for power, if this person only wants background music, a receiver might suffice. Background music is only going to draw a few watts, and one assumes she won't have all the speakers turned on at the same time.
A pro amp for under $300 will give you 200 watts / channel. That would almost certainly suffice. Make sure the speaker box is rated for this.
I have never needed to deal with any such setup, so this is my best guess.
You might want to contact Niles directly if possible, or find a place which not only sells the boxes but understands them (good luck with that
Also note, that you could run a more complicated setup using speaker zones. But I would not reccomend it for this person's needs.