L.T.
I was going to respond in much the same way as Denise. Keep open communications with the mother and the children. Restate the rules and consequences and be consistent with adhering to them.
I want to mention one other thing that is sometimes hard for us as parents to remember. I worked in a daycare center a few years ago. Part of my training focused on recognizing and dealing with "age appropriate behaviors". These are behaviors that any child of a particular age could exhibit. Sometimes they are unwanted behaviors - biting, hitting, telling false stories, etc. While they may not be "socially appropriate" they do happen and they are part of the growing and learning process. As parents or caregivers we need to be aware that these behaviors are normal but we also need to teach kids more suitable behaviors and let them know there are consquences for continuing the unwanted behaviors. Good luck!