If I'm correctly assuming that this will be a long haul flight, as others have said the toilets will be open as before, except for the first and last portions of the flight, covid restrictions or not. If it's short haul then toilet beforehand and minimal fluids during the flight. Your nervous flyer should try to reduce their anxieties, at least for the journey, as everyone knows that the waterworks will delight in demanding attention when we're stressed. I hope the journey will be stress-free.