The bathroom needs to be very warm indeed. You have to enter the empty bath, shut the door CAREFULLY, fill the bath, bath, then sit there while the water drains away afterwards. As you can't immerse as in a normal bath, I would suggest that they look at a level access shower cubicle instead. It will be easier to use and less chilly, also a massive saving on water compared to a walk in bath.
Level access showers are also much less disastrous if the door seal fails....and it does happen!