Can't really give you a solution because we have the same issue and haven't managed to solve it but can empathise - DD2 14 is an awful liar and has always been a problem since she was young. She will happily tell you the sky is green and the grass is blue and the fairy down the garden did it rather than tell the truth. She will lie even if the thing isn't something she is in trouble for, e.g., if you ask her what she had for lunch. She will continue to lie, even if you can prove that it's a lie.
She has in the past told awful lies about family members and her home life at school, and subsequently had to admit they were lies when the school questioned her about them as friends told the teachers what she had said. I have, in the past, had mothers of other children confront me about lies she has told about us, including allegations of abuse, and I have had to tell them that if they are concerned about anything she says, that they should take appropriate action and call SS, as I would love for them to come and see the truth of it.
She is a very 'skilled' liar, and on more than one occasion I have actually fallen for the sob stories she pulls about how she is the victim, and then gone on to find out that she has actually spun me a merry tale and none of what she said was the truth. She has no qualms with painting other people with awful brushes to get herself out of a pickle.
No one believes a single word she says about anything, which is really sad :( Despite trying to get her to see sense since she was 5, multiple readings of 'the boy who cried wold'-esque stories, etc, it has made no difference at all.
So, short answer is - you aren't alone, you have my sympathy and if you figure out how to fix it, please let me know!!