Before SB went, the Prevent programme, police, children's services and security services had been monitoring her internet use and were aware she was being radicalised. Rather than tell her parents, they gave her and her two friends letters to hand to their parents informing them of the concerns. The three girls were flying to Istanbul and onwards to Syria within days.
As others have said, the UK can't remove her citizenship, international law won't allow the country to leave her stateless. The UK does have a choice about whether it sends the military or any other personnel to help her escape Syria and the camp she's living in. I agree with the spokesman I heard earlier who said why should anyone be put at risk in that way
If she manages to get back to the Uk I hope she's arrested, remanded and subjected to psychological and psychiatric tests. She has given birth to two children who later died. I can't imagine what motivated her to go and imagine that whatever distorted belief systems were behind her actions, won't have been helped by exposure to the horrors in the caliphate. We don't know if she was involved in any of those horrors, whether she was involved in the awful abuse of Yazidi women and children.