People should be allowed a chance of redemption, but if she believes that about the Manchester bombing, fuck that. She obviously still has radical beliefs so probably best to exile her.
Emotionally I can see where you're coming from with this mate. But it's not so much about "redemption" as it is about deradicalising a British Citizen who, like it or not, by law we are responsible for. She won't come home and be allowed to crack on as if nothing ever happened, she'll be tried for any crimes she may have committed and put on the path to recovery.
She was 15 and old enough to know better but ultimately she has been a victim here. Not nearly as much of a victim as those who were lost at Manchester, or the families affected, but she was very obviously targeted and brainwashed while she was a child. This is how those wankers get you. Like Danny Dyer said (I never thought i'd start a sensible reply with that) at some point down the line she decided that flying off to Syria and leaving her entire family behind was better than living in our capital with all the freedoms that come with it, the chance to go to college, university, become anything that she wanted. That is not a decision that someone with a sound mind makes.
She may well still believe that the Manchester bombing was justified which wouldn't be a surprise given that she has been heavily indoctrinated and been living in Syria with terrorists for the last few years. If we turn our back on her completely then all we've got left is one more potential terrorist in the world (well two, if you count the kid).
To be honest the debate over the morals of it all isn't really that relevant, by law she has to be allowed back in the UK and that's the bottom line. It is a really tough one but if we start living outside of the law then we begin to lose some of the values that separate us from them. What the government needs to look at is how a 15 year old kid decided to make such a mad decision, managed to go through with it and what can be done to prevent it happening again.