Stacey Morgan, a 15-year-old girl from Swansea, has branded her so-called best mate a "crafty bitch".
Morgan said that her friend, Jenny Smith, also 15, pretended to like people but only to use them for things like "sweets and lifts home from school".
The claim was made in Morgan's diaries, which have been published today in a Sunday paper.
In one incident, it is claimed that Smith used a boy in order to get some chips.
Morgan wrote in her diary: "Today we were down at the shopping center and Jenny was being really flirty with Josh, even though I know she doesn't like him and she knows I do.
"She was all pretending to like him and holding his hand and stuff. But as soon as he bought her some chips from MacDonald's she started ignoring him.
"She's such a crafty bitch."
The revelations are sure to rock Swansea's youth scene, where it was thought that the girls were inseparable.