Hi and welcome.
I'm assuming your mother is Type 2 - there's a Type 1 tag on your post which is a bit confusing. You also say "diet only" and then say "diabetmin" (ie metformin) as medication. How much of the metformin is your mother advised to take?
The other thing that would be helpful would be the last few HbA1c values. The fingerprick results can be informative but hard to interpret in isolation.
Has your mother tried a low-carb approach? I found that greatly reducing my carb intake via food had a rapid effect on firstly my blood glucose levels, and over a much longer time subsequently, my bodyfat. Others on here have had similar experiences.
It's worth reading around on this forum - try the "success stories". There's no set way to do it - you find the way that works for you.