Two years ago my daughter (now 21) dyed her thick healthy blond hair red. She didn't like it and with a friend put various blond/bleach kits on it until it was blond again. Now her hair is becoming thinner all the time and I am devastated it is so thin. Will it ever grow back or is it likely she has permanently damaged follicles?

The hair will grow back. When hair becomes damaged, it breaks off above the root -- the hair follicle (the root) doesn't get damaged.

It takes a while (hair grows roughly 15 centimeters -- 6 inches -- per year), but don't worry.