My daughter taught fifth grade for years and was forever talking about the girls - hormones kicking in, feeling bad for themselves and angry at each other - just the whole gamut of emotions.
Your daughter's best friend's boyfriend (that's a mouthful) is, most likely, a passing fancy on the part of one or both of them. Your girl needs to whoa up, back off a bit from her love-smitten friend, and see how long this relationship lasts before she decides it's worth screaming about.
You might also talk to her about the nasty word "JEALOUSY."
This is a time when everybody is comparing him/herself against everybody else. They wonder if they measure up. Measure up to what? Sometimes they don't even know. Your daughter may not realize it, but it's often the most "popular" kids, the ones who seem to have everything going their way, who feel worst of all about themselves... when nobody's looking.
There's still a lot of summer to go. Tell her she needs to use that emotion in a worthwhile manner. She needs to be involved in things - sports, crafts, camping, books at the library, trimming the shrubbery, cleaning the garage, doing roof repairs on the house (well, maybe not that) - things that keep her busy and will even, perhaps, keep her happy, whether she likes it or not. Tell her she must get busy! Growing up means using your body and your brain, not just your feelings. Tell her how *you* learned that (or how her Aunt Bessie learned that, or someone outside herself).
As far as her looks are concerned, let her know that everybody is fighting that battle. She can learn how to do her hair differently or choose her wardrobe, and have fun doing it! But when it's really boyfriend time, a worthwhile boy is going to like her for more than those things. Boys are usually interested in girls (if they're interested at all) who are *interesting* - that is, girls who get past themselves in their own minds, who know how to do things, who like others, and who aren't scared of talking with people. (And the boys are usually not that interesting themselves, if truth be told.)
This lecture is not for your daughter - it's just to give you ideas if you need to use them.