We aren't allowed to give dosing advice but has your DN said anything about reducing the tresiba some more if she keeps going low overnight?
I suspect that when you are diagnosed at her age long term complications may not be too relevant, and hypos are more of a concern than highs, but a blood sugar of 20 would make me feel unwell, so getting the insulin right so as to avoid the big drops would seem to me to be pretty critical.
Can you talk to the nurse again and get some input?