Hi, Welcome to health care magic, Severe diarrhea since last evening whatever may be the cause which may be drug induced or infective,(gastroenteritis) but it should be treated promptly before patient may go in hypovolemic shock (tachycardia, hypotension,rapid breathing, giddiness, fever etc). However,there is nothing to panicked here as your father in law had 3 watery stools without vomiting most probably i guess. He must undergo routine blood investigations and electrolytes,liver and kidney function tests,urinalysis and stool for blood,abdominal ultrasound to detect the underlying cause if any. So,he should be given antibiotics (Norflox-Tz is preferred),anti-spasmodics,antacids,probiotics with fluid and electrolyte replacement therapy. Here,it would be best not to wait for more time rather he should immediately consult some local or family physician and visit to ER. Thanks and regards,