That is one of those things that is hard because at that age the child can't be expected to always remember to tell you in time for the bathroom even though they do most of the time and especially if distracted such as sitting for tv, story, etc or tired as in nodding off in the chair. Also it comes down to what you let the child do. A child that age in underwear with less than a month of dryness probably wouldn't be allowed to sit on the chair at my house. I also have covers on the couch and they can sit there (I got what is called quilted table runner from fabric land and tuck it around the cushion and then put a folded blanket on the couch)

I probably would pay the bill myself the first time but mention it to the parents and then discuss that if it becomes a habit then they will have to foot the bill unless they want the child banned from the couch, chair, carpet areas. But it comes down to you taking the child more often to the potty and watching for distractions.

All kids stay in pullups here till they are telling me they are consistently telling me they need to go. I would rarely take a child out of pullups before age 2 just because they are too easily distracted, still deal with teething issues, a multitude of illness, sleep disturbances etc that makes it hard to make them responsible all the time.