How can I tell her without losing my friend?

[Tiggy]

My friend Grace is totally obsessed with Ariana Grande or Cat from Victorious the Nick program. She follows her on twitter and tweets her so much that she managed to get her to follow her and now Ariana tweeted her and yesterday DM'd her. In school this is all she goes on about I sit by her in most of my lessons and she will be like "Has she followed you yet?" but in a snide voice and she constantly changes the subject so i will be talking to her about something actually important and she will change the subject to Ariana Grande again. it's not just me who feel like this either it is almost our whole friendship group and she makes me feel bad about myself because she will be mean to me and if i say anything back she will yell at me for being me to her. she is super controlling of most people and she cannot understand that there is a life outside of twitter.
i don't know how to tell her because i don't want to lose her as a friend because besides all of this she can be really funny and nice. like she has stuck up for me before its like no one is allowed to pick on me but her.
and she is really self obsessed i will say things like "wow its hot in this classroom!" and she will be like "yeah cos i'm here" and then i will be like "its cold in here" and she will say that its because of my heart. i have tried telling her that i don't like how she is always picking on me and she just tells me to stop over reacting that i am meaner to her than she is to me and that she was only joking and that i should learn to take a joke. all the people in our friendship group want me to confront her because i am the closest one to her because she doesn't have that many friends but every time i try she tells me that i'm being ridiculous. but she tweets Ariana like crazy Ariana will say "Goodnight sweethearts!" and Grace will say "Goodnight Ariana. Sweet dreams, I hope your dreams are as awesome as you!" and i told her today that its a bit strange to tweet someone you don't know that and she says its not cos she says it to everyone but i have known her for 4 years as a really good friend and she has never said that.
she also has about 200 Ariana fans following her on twitter and she is constantly on her phone in school talking to her "friends" who live in America and Africa and she says things like "Good morning Beautiful" to them which is really strange and it seem like she is blocking out her real friends and the real world for her twitter friends and the twitter world. i have twitter and use it daily but she reaches her tweet limit everyday.
when i mention that she is going a bit over board with this obsession she just tells me i'm jealous because she follows her and not me but i'm not i am actually fed up of her going on and on about a person she has never met. she always has new obsessions but this one has been the worst she doesn't stop thinking about her she even wrote Ariana Grande on her pencil case.
All of her obsessions have been girls so people are saying that maybe she is a lesbian so i asked her and she said that she has obsessions with girls cos she can't find any man who is talented and that she doesn't want to have an obsession with someone just cos they are hot, but yet she always says that Ariana is breathtakingly beautiful. if anyone says anything slightly off about Ariana she goes off on a crazy rant.

i just wanna know how to tell her to stop because she is losing everyone but i want to say it without hurting her feelings because i really don't want to lose her. <3 thank you

:]
Btw we are both 14 so pretty much too old to be obsessed with someone off Nickelodeon but if I say this I get yelled at again.