You make the assumption that the problem is a power drain -
But that may (very probably is), or may not be the case - perhaps the alternator is not giving enough output?
Or perhaps the car is used in a way that the battery is flattened and it does not get a chance to recover (short trips, lots of starting, heavy electric use)
Or the engine is difficult to start (faulty glow plugs, lazy starter),
I recommend you get a plug in LCD voltmeter -
and monitor/ record the volts, dayin, day out - eg, if you park up on Saturday, make a note of the voltage, then see what it is on the Monday, before you start it (and after)
After checking that, you could charge the battery on Sunday as a temporary measure
Is it a smart charge system and a silver calcium battery?