Another explaination could be this.....
Claire has not accepted her mothers true vegetative condition. She still speaks about her as if she were in the same mental condition as before the crash. They were arguing in the car before the crash and possibly Mum was angry enough to be disowning her at that time. Or the boyfriends reference of "she already has" is referring to her current state of coma. Also if you notice at the end when she visits her mother she speaks to her as if she can hear. Such as "did you have a nice nite?" and I haven't told you yet but I should have before now........etc. She was not living with the reality of the facts.