My 16-year-old (ADHD, found not to be on the spectrum but certainly has some traits) is still like this to some degree. She displayed stalkery behavior toward one friend when she was 11 (didn't have a phone but would email the friend constantly and berate her when she (the friend) tried to step back - I only found out about this later or would have put a stop to it). At 16, she has learned she has to moderate her impulses to be in constant communication, but that doesn't stop her from privately obsessing about the friend all the time. In retrospect, I wish I had talked to her more about degrees of friendship and how not everyone has to be your one true BFF. But it is something we're still dealing with - it's hard for me and often sad for her.

I highly recommend the book “growing friendships” for her to read. It would help her understand boundaries in friendships and it is written for a young audience.