B.S.
Growing up I was like your second daughter. I had a friend who was my everything. We lived down the street for a long time until I moved but we still remained close. We had our ups and downs. Specifically one other friend who would come between us and I would be forced to do things with others, but I made it through. I would not worry about it so much your daughter will be ok.
As for my daughter who is 7. She has been SUPER close to one of her friends since preschool. In fact the two girls switched schools together. However, last year things weren't very good for awhile. My daughters friend kept leaving my daughter out of things and kinda moving on from my daughter. Even though when it was the two of them they got a long great! It was at this point when I had to start arranging some play dates with some other girls. And now continuing on into this year I find my daughter is having an easier time in school because she's learned to take in a group of friends instead of just one close one. (I encouraged this only because of her problems with her close friend and I figured having a group of friends allows you to fall back on others if need be)