I did like the book, I would say it's for any age group..:.but....personally if she likes Stephen King (as I do, I'm 25 and a massive fan, nothing wrong in that 
) I would probably recommend something else....
The Passage by Justin Cronin is very good. It's a trilogy with the 3rd book coming out the beginning of next year. It's kind of an apocalyptic vampire story - the stand meets Salems lot kind of thing.
Or maybe one of Gillian Flynns other books (I'm sure you've heard of or seen gone girl). Sharp Objects and dark place is fantastic. They're both thriller types but very dark.
There's also Wayward Pines, this is more of a sci fi thing, maybe along the lines of the tommyknockers and under the dome by King. It's also a trilogy set. These are written by Blake Crouch.
There's also something called Cry baby, this is one of the better thrillers I've read, stands out in my mind. It's by David Jackson
Also as a final thought - Mark Edwards. The magpies and because she loves me. Very good thriller stories.
Just my tuppence worth seeing as me and your mum both share a love of SK 