It's a really long story.

I'll try to shorten it up.
She basically made her rounds on our group of friends. She found one person she had a lot in common with at the time and latched on for dear life. For me, it was Freshman year, we both were on the tennis team. So we became pretty close, but as soon as I did something without her, she freaked. She would accuse me of "not liking her anymore", etc etc. At the time, I did like her and I valued our friendship. She was a realy good friend, she would always listen if you had a problem. But then I got sick of her always having to be near me. It was suffocating. So then we just kinda grew apart (the only thing we really had in common was tennis).
So then she became super close with my friend, "J". They were best friends for a year or two, then "J" got sick of her latching on too. So basically, at the end of this past school year, there was a huuuge fight between "J" & her, which basically turned the rest of the group against the clingy friend because she started spreading rumors about the fight to other kids in our grade.
The clingy friend was really, really dramatic though. I don't know if your friend is. If she is, WATCH OUT!! lol it could get ugly!