Good morning sorry to know about her illness Any mucus diarrhea in such a patient needs further evaluation so dont hesitate to bother your doctor your mother has compromised immunity due to her cancer, chemotherapy and probably a low nutrition. This poses them to risk of infections that can cause mucus diarrhea I were your doctor, I would immediately analyse her stool specimen for infection, parasites etc. and do a culture I wont hesitate to treat her with antibiotic course and if still persistent, I would go a step ahead and perform colonoscopy to figure out the diagnosis Hope this would answer your question Wish her a speedy recovery