Jeep Grand Cherokee 2003 2.7 CRD LTD Auto

I have a 2003 Grand Cherokee and for 3 months it has just been stored on a yard at work, I wanted to take it for its MOT just next door to the yard and then sell it. When I went to start it the battery was flat (battery only 7 months old) so I tried to jump start it from my daughters ford fiesta using my heavy duty jump leads, but the jeep was having none of it. So i went next door to the garage and asked them to come with their jump starter pack, they sent their young lad bradley. When Bradley saw the Jeep he said he should have brought the bigger pack as he only had with him a small one. He tried the small jump start pack but it didnt work at starting the Jeep, then he had an ‘idea’ he connected the heavy duty jump leads of mine to the fiesta and then the Jeep, then he attached his battery charging, jump starting pack to the heavy duty jump leads of mine and when he turned it all on there was a bang and loads of smoke coming out of the engine, the leads from the battery exploded and the lad was blowing furiously at the smoking part to the right of the battery. They charged me for a new battery, did the MOT and when I drove it home the dash board started spazzing out all the lights were coming on and off, the car lost power, the lights went dim and i pulled over to restart the car and the NEW battery was flat. The garage denied that Bradley attaching both jump lead and power pack to the battery caused any problem, he said this ‘ the leads smoked because it is such a large draw on the leads’….what do you think?