What Causes Recurrent Vomiting After Having Food?

My sister drinks at least 4 diet cokes, and 5 1 liter bottles of flavored/carbonated water per day. She eats mostly cheese sandwiches, some salads (2 per week) and is on major pain and depression meds (opioids) to the tune of 60-70 mg of morphine daily (15 mg at a time) taken with 2 oxycoton for a degenerating hip. She recently moved here to AZ from San Francisco, and lives with my husband and me. She has recurring vomiting attacks often right after she eats, and then spend the next several days flat in bed to recover. She gets no exercise, uses a walker to get around the house, and says she is always in so much pain. I do not know what to do to help her. She has high blood pressure and weighs 310 pounds.