Lying is very common in adolescence either to cover up or minimise rule-breaking or to boost self confidence like your teen is doing op.
I am not an expert but according to my vast library on teen issues (don’t ask!) constant lying to peers can be a bit of a red flag because in some cases it can be a sign of certain mh disorders In teens eg depression. So your instincts are spot on op and I would definitely pay closer attention to her and definitely help her see a licensed psychologist who specialises in teen issues.
A lot of the time though they just need reassurance that they are “normal” and are “good enough” and are experiencing similar worries and fears as the rest of humanity, but occasionally they may need more help.
For example, if the psychologist diagnoses anxiety or depression then that, plus friendship issues, could in some instances indicate autism. (Just an example.) And as a PP has indicated, masking and trying to copy others is classic behaviour for girls with asd, which is often diagnosed later in girls precisely because they mask. Or she may just need help managing anxiety.
I know it’s frustrating and you feel upset on her behalf because you feel she has ruined the opportunity to make a new start. Also it feels fundamentally wrong of her to lie so blatantly. But she needs sympathy and support not confrontation. By all means tell her she is a complete wombat for lying because the truth will always come out eventually and that will alienate others even more.
But if she has asd she may not be able to fully appreciate how being lied to feels from some else’s perspective. So what appears to be lack of conscience could be total lack of comprehension. So explain that and how it impacts on trust. It may help to draw a circle of trust visually as you would for a younger child (imagine people holding hands and standing in a circle) then each lie is represented by rubbing out a bit of the circle, leaving a big gap.
Obviously check with school that she is not being bullied and talk to her teachers about her friendships and the way she interacts with others. How is her academic work? She obviously needs loads of reassurance that she is as good as anyone else, that no one is perfect, that we all make mistakes, and that it’s ok to show some of our imperfections to people we trust. How other people find it endearing when we are honest about our difficulties.
Explain that, as adults, we become mature because we are more practiced at things but also by being accepting of, and reconciled to, the fact that we will feel uncertain and inadequate at times and that’s ok.
Also if you can get her involved in a hobby outside of school to build her self esteem that would be good. Something related to an activity she is naturally good at. Having separate friends outside of school can be a game changer for a teen too.
If she is sporty and independent then the DoE awards might be good for her but anything from horse-riding to arts and crafts will build her self-confidence and boost her sense of identity .