"The Best Friend (Fear Street #17)" by R. L. Stine

FEAR STREET -- WHERE YOUR WORST NIGHTMARES LIVE... Honey Perkins just moved to Shadyside. But she's telling everyone that she is Becka Norwood's best friend from elementary school. Trouble is, Becka doesn't remember her at all. But that doesn't stop Honey. She insists on doing everything Becka does -- borrowing her clothes, borrowing her boyfriend...and then the horrible accidents begin. Honey swears she has nothing to do with them. She's just being a good friend. A best friend...to the end.