Thanks JK. The plumber repaired meaning they unclogged the toilet with a snake. According to the plumber he encountered resistance on the segment of the drain pipe that runs from the toilet bowl to the main drain pipe of the house. I.e. there was something big enough to clog the large diameter pipe and it was located in the section of pipe coming directly from the toilet and not from any other location in the house. The tenant denies that they placed anything in the toilet that could have clogged it. However they also had friends over that night. In my opinion it's pretty clear here that they clogged and are responsible for the clog. What isn't clear is who is responsible for the valve not working and leading to the overflow.