Have you considered disabling the touch controls on the speaker?

https://support.sonos.com/en-us/article/use-button-controls-on-sonos-products

I have, but she uses them to start/stop playback, and skip songs. She has no devices with screens, so the on device controls are her only means of controlling the speaker. 

70% is still pretty loud. Have you tried setting the limit much lower like around 40%?

first I need to get the limit to actually function. Then I can figure out what limit is actually appropriate.

How do you know it isn’t functioning? 70% volume limit turned up to the max is still extremely loud.

Stream music on the speaker and adjust the volume limit from 100% to 25%. You should hear the volume decrease in real time. If it decreases, it is functioning as it should.

It’s actually going to 100%. It shows in the app as 100% after she turns it up. It is the same volume level as 100% with no limit. 

Just because you set the volume limit to 70% doesn’t mean the volume slider stops at 70%. It will still go up to 100%. 100% will just now equal 70%.
 

When you adjust the volume limit from 100% to 25% while music is playing, do you hear the volume decrease?