It could be so many things, but naturally our minds immediately think of the C word.
If it is any consolation to you I had similar symptoms, blood (dark) in stool, rapid weight loss, exhaustion, the works. Naturally I prepped myself for bad news.
I think I had every scan, ultrasound, endoscopy, colonoscopy and bloods that they could think of, the hospital consultants and staff were brilliant.
Turned out I had Helicobacter Pylori which resulted in a microscopic bleed in the stomach from a perforated ulcer. That had been going on for a long time asymptomatically. So the black stools were the result of the upper GI bleed. Sorted with Nexium type drug. I had big polyps on the bowel too, which were removed via colonoscopy. Got a blood transfusion as the internal bleeding caused iron deficiency anaemia.
Touch wood I'm fine now. Check up once a year for stomach, and three years for bowel. It is not always bad news, but I had myself in knots of worry, it is natural.
I hope everything turns out ok for your relative. All the best.