Metformin can cause problems with vitamin B12 uptake, so you might consider a supplement of that in addition to a general vitamin and mineral daily supplement.
I take one without iron, as I am eating more meat than the normal non diabetic and I am approaching 70 years old, so things naturally work more slowly, or so I am told.
The usual diet eaten by tpe twos, which is low in carb but has enough fat to compensate, is very nutritious, and I only need to eat twice a day as a result, so you mum is not necessarily undernourished. I must confess to taking only a couple of the multi stuff tablets a week but I eat all sorts of plants and the tablets are more to compensate for a general reduction in the soil rather than a lack in my diet.