My Dad Has Had Vomiting Which Is Brown In Color,

My dad has had vomiting which is brown in color, constipation, dizziness, weakness, sweating, and is pale. Symptoms for 2-5 days. he is in his mid 70's has untreated prostrate cancer, a pace maker for 3 years, a smoker, and consumes alcohol regularly. Refuses to see his doctor as it "is just a bug" . What possible conditions does this suggest. I recently had similar symptoms that went away in a few days. Also have known others in the area who have had same. Could this just be a stomach flu? What signs to look for before going to the E.R.