I saw the documentary. It was good, and I didn't change my mind - I felt that she was groomed and in any case too 15 is too young for anyone to be held responsible for a stupid decision for the rest of her life.
She isn't very likeable, but that shouldn't be a good reason to deny her citizenship. There is no way of knowing whether she is traumatised by what happened to her (she was married off to an abusive husband, sent to live with a man known to the authorities for being violent whilst her husband was in a toture jail, and had five pregnancies, two miscarriages and three dead babies), and I don't think that she would get a fair trial anyway - it would make more sense to have her case put before a panel of experts, such as psychologists and agents specialising in terrorism.
The longer she is denied entry to the UK, the older she will be (obviously!) and the more likely it is that she will be 'got at' in the camps, whether that is to attack her or recruit her to the cause again, if she left it as she says.