as the others have said, she is lucky to have you for friend...
my son also had caring friends but in the end they couldn't 'save' him from his addiction....
I couldn't either but I do wish I had known some of the early signs so that i could have offered help before it became a crack/heroin addiction...
what your friend is experiencing is common...drug craving that includes vivid dreams and trigger recalls (such as thinking it is in the book etc...)
this is a common path to relapse....a relapse doesn't begin when an addict picks up the drug...a relapse begins when an addict lets these cravings,dreams,triggers continue...
your gf needs support from a counselor/ therapist /sponsor etc...
you need to take care of yourself too
it is so easy to get sucked into this disease and it is an ugly place to be
please encourage her to get support...
if she spent time in rehab she knows that she needs support...refusing to seek help is often a passive way to seek drugs
in the end its up to her but you get to decide what your life will be like...please learn all you can and be careful!!