This is a practice that's commonly done by airlines in the USA and by FAA regulations, the remaining pilot on the flightdeack must wear their oxygen mask when above FL250. It also begs the question whether the flight attendant who enters the cockpit to accompany the remaining pilot can legally sit in the control seat of the pilot who has left on a lav break? Standing on the flightdeck is not a problem when the air is smooth but if you encounter some light or moderate chop, especially in the smaller cockpits like in the 737 or CRJ/ERJ, surely you'd want the FA to be seated?