How can I tell her I don't like her?

[Gemma]

similar thing happened to me, but after a while my friend realised how annoying this other girl was. so yeah, if she's as annoying and horrible as you say, "chelsea" will soon realise it too, and then she'll wonder how she didn't see it sooner. so try push through it for a while longer, and then i'm sure things will be fine.

[Narnat,]

I think it is important for you not to be seen as ganging up on her when you say something so maybe make it straight to the point and walk away if she follows well be a bit harsher. If she still doesn't get the message get Chelsea to do something because she is her friend. Good luck!