The parents need to be on the same page as you? Tell the child straight out that they are not allowed to poop in there pants at your house.
Tell them at your house its strictly on the toilet for pee + poop, ask them if there a baby? Say babies where diapers and your a big girl at my house so if you go in your pants then that means your a baby and diapers must be worn. Seriously be firm and to the point. It worked for me with a daycare girl last summer.