Most of what I remember is in the subject. The 'terminal patient girl in hospital' setting is just a prologue, so to speak. She persuades her (male) best friend to go into this new world with her ... it is not clear from the beginning of the story whether this is a dream or an alternate reality or what. I found the story again some time ago but I can't seem to find it again as of this month.
Suggestions, etc welcome.