From your last statement it certainly seems like the starter motor is drawing too many amps and lowering the voltage too much but I guess it could be something else causing low voltage and the starter is just tipping it over the edge.
My starter had an internal short at about 65,000km after a trip to Cape York, so it is not unheard of. However, mine couldn't turn the engine over and resulted in smoke and very bad smells when you tried to start it!
EDIT: They should be able to measure voltage and current draw on the extra battery to confirm that the starter motor is drawing excessive current before removing the starter.
				
			 
			
		 
			
				
			
			
				Bob
2010 D4 3.0TDV6 SE, ediff, LLAMS, 5 x GOE wheels, LT285/60R18 BFG K02's, GOE Compressor Guard, LR Tank, Mitch Hitch, ECB Bull Bar, Kaymar Rear Bar, Traxide, Safari Snorkel.
2019 Discovery 5 SD6 SE, 20 inch wheels, 275/55R20 Nitto Grappler G2 tyres
			
			
		 
	
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