"To tell the truth" is not what the oath says: It says to tell the truth, the WHOLE truth and nothing but the truth. It was written that way for a very particular reason.
"She and her boyfriend may be guilty of a crime, particularly because she lied and he knew it."
Isn't it possible that she was raped before the party... by the boyfriend. Perhaps that's why she was so upset. She might have been upset enough to let it slip to the police that she had been raped, but then not have wanted to tell who. It would explain how distraught she was and why she kept changing her story. Just a theory though!