If the parents continue to bring her in "inapropriate" attire for potty training, I would just ignore their lack of response, have a larger (not too big, but big enough to be loose around the waste and easy to pull down and up) pair of pants kept in her cubby. When they drop her off in the am, just change her out of the cute duds they bring her in and into the cozy/easy ones you have. That is just what I do. When she is at your house you can do what you choose, what works best for you. Maybe if you get lucky they will make a comment regarding her having accidents while trying to pull pants down, and then you will have a chance to explain to them the importance of ease of ups and downs with pants while training. Not to mention independence is a huge part of successful potty training.