Poor thing...
You need to keep an eye on her blood glucose levels, as the virus is likely to put this up, and check for ketones frequently, adjust insulin dose as requried, if you'll not confident in this get hold of her diabetic team for further advise..
So general sick days rules for blood glucose control..
I would also keep a very close eye on the blisters, the fluid in them, should be clear and watery looking, if this changes then it's probably a secondary infection setting in which requires antibiotics.. Also when they start scabbing and she starts scratching keep a close eye on them for infections brewing..
My oldest daughter had to have antibiotics for chickenpox, and she wasn't diabetic!
Plenty of fluids..
Carolime (sp) the pink stuff, is very good for soothing the itching of chickpox and because it dries pinkish colour, you can actually see what skin you've covered or not..