How reliable are tests done at home?

My mother uses one and rings me to tell me her
sugar varies from 100 one day to 200 the following morning.
She says because she had half a mango.
On the glycemic chart a mango is well within the range.

I have read articles where doctors regularly claim that diabetes
can be controlled and cured with diet alone - if one consumes
foods with low glycemic load.

Has anyone had any experience in this?

Jeff