It sounds like the float inside of the tank is not shutting the water off.
The red arrow in the picture above is pointing at the float, this controls the water flowing to the tank. As the toilet flushes and water runs from the tank, the float lowers and turns the water on to fill the tank back up. When the float gets to a set point where the tank is full of water, then then the water will shut off.
It is possible the when the customer was installing the t-valve, they might have moved the fill valve and the float might have gotten stuck on something. I would have the customer look at the tank and check to see that the float is not being blocked or stuck by any hoses or anything inside of the tank. They can manually lift the float to get the water to stop and that might help them to determine what is causing the float to stay down.
If there is nothing blocking the float from rising, then the entire fill valve may just need to be adjusted or realigned. It is possible to very gently turn the fill valve clockwise or counterclockwise to adjust the position of the fill valve and get the float to rise back to its normal position.
Please be aware that there are two types of floats, one is like the picture above and one is a ball that is attached to the end of a wire, like below:
You will generally see the ball float on older style toilets.