i was in this same situation but the other way aroundd yesterday, i said i cant pick.
if she really loves you as a friend she will suport you.
i dont believ in chick before dicks.
thats bullshit.
let it go she will come around, just tell her how you feelo about her and tell her you dont wanna loose her. that might work. thats what i did.
dont worry about it girl things will work out.

Your "friend" should not blackmail you. She sounds jealous. If you want to date the guy you should. If your friend doesn't like it and is willing to end your friendship because of it, she is NOT your true friend.

chicks before dicks.

She doesn't sound like that much of a true friend if you ask me. :l

I'd talk to her about it beause that seems a bit rediculous. Imo, if she's a real friend she would want the best for you and wouldn't put you in this 'him or me' position.

Well in my opinion I feel that your best friend is kinda jealous . If that guy is handsome then maybe sshe doesn't want you to date him.It maybe because of two reasons
1) She has a crush on your boyfriend
2) She thinks that because of this guy you might not be so good friends anymore. In simpler words , you two might just get seperated .
Whatever the reason, if she's your friend go and ask her . She will definately tell you. If that does not happen then maybe you have chosen the wrong fellow to be your good friend. Friends don't behave that way.

Are you sure your "best friend" doesn't have a secret crush on him?

Honestly when people around you start with the "it's either him or me" crap dump them. Don't stand for the nonsense. Sounds like and insecure and jealous girlfriend. If she was truly a friend she would not have said that, she would have explained why she doesn't like/trust the new fellow and then stand by and (maybe) pick up the pieces when it all breaks up - or be your chief bridesmaid at your wedding cheering you on and being there as you would and should be for her. But a best friend never pulls a stunt like "me or them/him/her".