Thellim is already fasting when Isabella calls. Earth makes it harder to stay healthy, she has some fat worth burning down, and the whole business with only food eaten in the last 48 hours "counting" makes extremely little sense to her, so she is fasting extra-long just in case the rules are different when you're as old as she is.
Thellim hasn't done very much good for Earth yet. But there's so much she could say if she just started blogging one post per day about it, and it's not a forgone conclusion that nobody would listen. From a utilitarian standpoint the correct decision is very clear.
[I don't intend to kill myself. But if I do, please try to call my mother and dath ilan anyways, now and then. I don't think it will ever work, but please try.]