I am 36 and almost ashamed to say that I am also afraid of bright sudden lightening and a loud crack of thunder. It nearly sets me into a panic attack. My fear came from when I was little my sibling brothers would scare me even more if there was a storm. I don't like sudden noises now or am easily made to jump. Maybe she could turn a light on by her bed so she doesn't see the lightening so bad and maybe sit with her if it's bedtime and chit chat with her as it's happening to help soothe her.
My 16 month old amazes me when he sleeps through even the most severe storms while I am very much aware of what's going on. I know it's not much help but I know the light has worked for me in the past since I didn't have anyone that would sit with me when I was young and scared.
The other thing you might try, go on the internet or to the library and get books or video about what makes lightening and thunder. I am very fascinated to learn about the planets and the solar system along with what I have seen on storms. Hopefully you can find material that is obviously not graphic and frightening but once she really knows what's going on, maybe it won't be so scary.