Zero button "0" is adjacent to 1-9 numbers, next to "6" and just below "Kitchen Timer" button and above "clock" button. View photo of product on website to verify.
Your mother can press "power", then "1", and then "0"three times to set it for 10 minutes.

The zero button does have the words 'kitchen timer' over it, but when the number 1 is pressed, followed by three zeros, and then the START button depressed, the microwave begins cooking a 10 minute cycle.

Perhaps she is in the wrong mode? Maybe in the timer or clock mode? Try having her press the Stop/Clear button, then 1000, followed by START. Or try 9:59. Good luck!

There should be a grid of number 1-9 to the left of the zero button. She needs to press the 1 button then three zeros to get the microwave to run for ten minutes.

My Sharp 1.1 cu ft. mid size has a 0 button just under the words kitchen timer. I just push 1000 then START so 10:00 (minutes) shows on the screen & cooking begins. If that does not work then it must be a "lemon". Hope this helps your mom. .

I actually do not use majority of the features. I set the 'clock' & then only use the 'popcorn' button; the '30 second' button & also use the 'numbers' to cook Lean Cuisine frozen dinners which is usually 7 minutes... My zero does work & just push 700 .... I just looked at my book & says I have 1) Display; 2) Auto Cook; 3) On Touch Cook; 4) Defrost; 5) Time Set Pad; 6) Power; 7) Clock; 8) Stop/Clear;; 9) Start/+ 30 Seconds. As mentioned, I only use a few of those... Sorry I couldn't be of more help, however, as a suggestion perhaps have a handyman or friend of hers take it back & get a simpler one..... Good Luck.....