It's brainwashing rather than grooming, I think. And it's presented as being true Islam, waging Jihad against the Infidel. Moderates see the word Jihad as being metaphorical for our personal battle against sin. That's as I understand it, I'm happy to be corrected if I'm wrong.
In addition, the Jihadists can claim that they, and not the moderates, are being obedient to the Quran. (The Bible has been used in this way, too, historically.)
15 year olds tend to see the world in black and white terms. They're being offered the chance to be part of the glorious war to make all the world part of the Caliphate and subject to Shariah Law. If they become a martyr, they will be assured of a place in Paradise, or so they're told.
For a teenager without direction in their life, that would be pretty seductive.
I'm not saying that justifies what she did. But demonising her doesn't get us anywhere. She's hardly the only British Muslim to be sucked in by IS. Saying they're all evil is too simple, as it absolves all of us of any responsibility.