She did not ask or expect "consular support" - she is now demanding her right as a British citizen to return

You obviously didn't read my response: If she were able to travel to the UK, and present herself at customs it is possible that she would be taken into custody, and then dealt with under UK law processes.

Your lack of humanity and a sense of justice disgraces you as it has Britain
My 'sense of justice' is somewhat old-fashioned. She has made her bed, let her lie in it.
And before you go on about her being only a child (15) when she went out, she has grown several years since then, and could have tried to return at any time. She has chosen to try only once it appears certain that she joined a losing cause.
There are many causes more worthy of my sympathy than this self-publicist who is suffering the results of the choices she freely made. If she travelled out under her sister's passport, surely this makes it clear that she should have known that what she was doing would be considered as wrong by many people, and by the law of the land.