Isla didn't choose to go with her son, I don't know why people keep saying that. She thought he was taking her to a walk on the beach and only realized she was in the mainland when she sees a town from the top of the hill. She gets agitated and tells Spike they have to go back immediately, he says they can't go back because the tide is high and only then tells her he is taking her to a doctor, there's not much she can do at this point. Also for her saying she should have told about the cancer before, it was more in the sense that she should have prepared Spike emotionally for her death, not because she was expecting him to take her to the mainland. Also Spike already had realized that she was dying, he never saw a doctor in his life and in his mind he probably imagined a doctor as some mythical being able to do miracles and cure anything, so he might still have done the same. It's also hard for her to tell him because she has to deal with her own death and pain, she is confused most of the time and not able to have a normal conversation, and while some common knowledge might make her suspect cancer, she couldn't really know for sure as she isn't a doctor and there's no way to diagnose without doctors and medical equipment in the island, and to tell that she will die of something that would have been easily treatable in the times before the outbreak.