This is only from my own experience (since I don't have any daughters and my boys are young), but I would start by reminding her that most girls struggle with friendship at that age. Lots of girls have aspects of their personalities (or looks) that makes it harder to make friends. And even the ones who look to be so happy and popular might be stressed about friendships. It could be that they feel pressure to remain popular or that their friendships are superficial.
And she's lucky she has friends. If she looks hard enough, I'm pretty sure she'll find a few girls who are outcasts and/or who struggle to maintain even one friendship. I know it's easy enough to say this, but a small group of pretty good friends is better than a large group of fake ones. Plus, it would help to focus on the positive in her friendships - just because they're not "like her" doesn't mean that they don't have wonderful things to offer.
And finally, even if it's not friendships, everyone struggles with something. I have my own anecdote about that: When I was 17, a high school classmate found me in tears in the girls' bathroom. She was shocked when I told her that my mom had just died and she saw that I was deep in mourning. I'll never forget her words to me: "But I thought that your life was perfect!". Nobody's life is perfect. It's all about how we deal with what we've been dealt. Attitude plays a large role in it.
