My mother was type 2 diabetic, and she probably did the 5:2 diet without knowing it. The problem was that my husband was type 1. We'd go for a meal and sometimes there would be lots of carbohydrate, the next time there would be none.
I think diabetesuk would frown on the 5:2 diet. Carbohydrates need to be balanced every day/mealtime so you do not have an overload or have less carbohydrate than you need.
The best diet to look at is the GI, or glycaemic index/load. It was developed for diabetics, but you would not know that now.
There is interesting research being done at Newcastle about going on a very low calorie diet to get rid of type 2 diabetes, but they reckon it will take 5 years for the research to have results. Your friend can ask her GP about it.