have they checked the volts/amps drain from the battery when everything is switched off. to do this you have to fit an amp meter between the battery and earth lead so all current flows through meter. then with the car locked and all electrics off check the reading on the meter [should be 0.2 amps or lower] if there is a reading then an electrical component has stuck on draining the battery
for eg a 0.5amp drain could be a boot light staying on
do not over load the rated load of the meter
happy hunting