You should always enforce your own policies. If they are not important enough to have, then don't have them.
It undermines your professionalism.
Second, he is pretty old to not be potty trained so I understand the frustration for both of you. How many times per day is he being taken to the potty as part of the daily schedule? Is he behind in other milestones? Has she told you of her plans for preschool attendance, yet?
Pull-ups over his cloth is not an unreasonable compromise.