Either type 2 or pre diabetic?
6.1 is at the bottom end of the prediabetic range, but there is nothing to stop her being slow onset T1 / LADA , though I agree at 71 T2 is more likely. (We had a post yesterday from someone whose 78 year old wife had just had a T1 diagnosis.) And forum rules mean that we aren't allowed to diagnose anyway, even if we could, so I wouldn't even say that she's definitely prediabetic.
When should metformin start?
That's really a question for her and her doctor.
She could also try lowering her carbs a bit more if she wants to reduce hba1c without medication? It's really up to her whether she wants some meds, less carbs or is happy to stay at the current level. Also, some doctors are happy to prescribe metformin to prediabetics, others may not be, so I'm not even sure whether metformin is an option here.