That is very sad to hear. Four months isn't very much time, but if she's not accepting any treatment, then she might not be as weak as if she WERE on treatment. Now one question: Does she have a chance? Or is she just too afraid to get treatment done? Like the whole disappointment of it not defeating her cancer or anything like that?
For ideas I would say talk to her. See if there's anything that she really would like to do before her time is up. Or try to see if you can get a program like Make-A-Wish to maybe do something for her that you yourself could never afford, etc.
In my honest opinion I'd say being there for her, keeping her company, and letting her know that she's NOT alone if the best thing you can do for her. I'm sure she's scared and having someone around to be there for her might help her with this.
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I love you!