2007 F550 work truck was dead. I gave the jump box to my employee so that he could jump it. This is what he said he did and what happened….
1. Hooked the box up to one battery and attempted to crank, and didn’t work.
2. Hooked up to other battery and attempted to crank, and didn’t work.
3. Hooked the + to one battery and - to the other, and attempted to crank which also didn’t work.
4. He said that when he tried it hooked up to both batteries, the dash lights were basically glitching out and flashing real weird.

When he came back in to tell me what happened, I noticed the jump box was dead. I thought it was full when I gave it to him, but can’t verify. Either way we charged the the jump box and tried again. Hooked the box up to the driver side battery and attempted to crank, and didn’t work. With the box still hooked up to the driver side battery, we see a nut on the terminal of the passenger side battery glowing red from heat. Once we noticed that we immediately disconnected the box and it stopped glowing. Held the cable from the box onto the driver side again and within about 2 seconds that nut started glowing again, so we stopped before we made it worse.

I don’t know why he decided to hook the box up to both batteries at the same time, but I’m assuming that’s what caused the issue. But what the hell is the issue? Where should I start as far as diagnosing the electrical problem he caused? Thank you