Your friend can try a hypoglycemic diet and see if that helps. She should eat every 2 or 3 hours from when she gets up to when she goes to bed. This would probably be 3 meals plus 3 snacks a day. All meals should have protein, low glycemic carbs, and healthy fat. Stay away from soda, bread, pasta, cake, ice cream, etc. Meals and snacks should be small so she doesn't start to gain weight.
Breakfast should be eaten shortly after waking up. She should have a small snack about an hour before bed. If she wakes up in the middle of the night with low symptoms, she can keep a snack by the bed. She should eat "by the clock" and not wait until she is hungry.
I've been living this way for about 20 years. I've had a lot of tests and nothing has ever shown up. I control my symptoms with diet and I always have food with me (nuts are great snacks.)