I will try to explain what may be happening .
If the parking brake shoes were of good quality and installed correctly, new shoes and new rotors don't always make 100 % contact . To help remedy this situation, the parking brake shoes must be worn down to where you get close to 95% contact between the shoes and inside of the rear rotor.
This is a common problem and those who don't know what to do throw up their hands and say that is all we can do They are parts replacers and not real techs, sorry about that.
The only way those shoes make any contact to conform is when the parking brake is applied and the vehicle is moving. I have done this many times and that is to get the vehicle moving 5-10 mph apply the parking to stop the vehicle. This is the only way to wear off the high contact of the shoe so it will eventually conform to the arc of the inside of the rotor.
If whoever does this procedure is not prepared to release the parking brake when the wheels lock up , they should be.
If you have done this a few times wait a while for the brakes to cool then repeat the procedure. You should notice an improvement after a few cycles.
I want to know how you make out with this because I would never suggest anything that is unsafe or if done exactly as suggested will not correct the problem.