Girlfriend has pretty deep mental health issues, she got one of these for Christmas and it works really well as a coping technique. Takes her mind off of whatever she's obsessing about and instead she gets lost in colouring. Highly recommended as a distraction to an overthinking brain.
I make colouring(with paint) books for kids on iPad and have been giving serious thought to make adult ones. Do you think your girlfriend would interested in working on a digital surface?
My (unsubstantiated) opinion is that the contact with a physical piece of paper (emphasis on physical) has much more impact than we think.
Personnaly I like zen doodling :).