Obviously she doesn't want to make an exception about her weekends (or that particular weekend, maybe she has plans already). You can be disappointed or angry, but it is her personal boundary, whether you agree with it or not. I know many people who would act exactly like her. And I also know people who would not (like the coworker you mentioned) or would be more flexible if there was an obvious crisis. People have their boundaries that are set for a reason. Maybe having a local therapist would work better for you?