She has gone to the dark side. Some girls like to conform out of insecurity. The popular group offers admiration, ego stroking, and (sometimes) base pleasures. She might have been envious of them and chosen to leap at the opportunity to join the group she considers “superior” or more fun in some way. I think it’s best to let go. If you fight for her friendship, she might misinterpret or react badly and reject you more agressively to show loyalty, or because she’s embarrassed in some way. That might cause a greater rift than this (relatively) slow drift. She’s trying to figure out what femininity means to her, hence the dress-up and increased interest in boys. If she realizes how silly and superficial the popular kids are, and remembers that she’s really a fun-loving dork at heart, (although it’s possible she won’t) she’ll come back. Very sad though. I’d say try to mingle with the rest of your group, as hard as it may be.