DD is 12 and off to high school after the summer. Over the past year there have been changes in allegiances between the girls in her class but they seem to have settled down into new friendship groups. DD is very friendly with one girl who I will call X. X seems a lovely sweet girl and has been over for sleepovers, on days out with us etc. They were allowed to chose who would be in their class at high school and DD and X are together. So far so good.
I have started to notice that DD seems to be at X's beck and call - eg this weekend X was going to come round but hadn't confirmed details. DD took phone into her room and refused to do anything else until she had heard from X. Eventually X phoned to say she was busy and couldn't come round. A lot of this sort of thing happening.
Anyway - I'm not sure if it's fair on DD now she's growing up but I often have a quick look at her email account. Got a bit of a shock yesterday. Going back over the previous month or so there are a lot of extremely controlling emails from X along the lines of 'you really upset me when you talked to Y at school today' type of thing. DD replying 'I hope we're still BFFs' etc and X replying with stuff like 'I don't know I'll have to think about it' type emails, keeping her hanging on with more and more emails saying stuff like 'I'll give you one more chance but you need to learn to treat me properly' etc etc. It was worse but I can't bring myself to go back and have another look. Lots of emails from DD grovelling 
They seem to make up then it happens again - I know this is normal to some extent for girls but the tone of Xs emails really chilled me
Should I do anything? They're 12 (well X is 11) so not sure how much I want to get involved but I want to warn DD that X is manipulating her in an unacceptable way. I can't help thinking it may get worse when they start high school with a class full other girls they don't know
Can anyone with experience of this type of situation advise? DD has managed to avoid this type of thing up to now - maybe we've just been lucky. Sorry this is so long!