I don't believe that diabetes and Metformin entitle you to anything by themselves. If she has disabling complications arising from her diabetes, then it may be different.
I claim esa and PIP. Neither are for me being type2.
From what I know with type2, that won,t be enough for either unless there are what I call, severe problems resulting from the diabetes.
With PIP, if your mum goes for that, then again your mum would have had to suffer for 3 months minimum with severe diabetic problems and would be at least to suffer for the next 9 months.
I get both benefits for something else