Possible reasons for a no power, no start situation?

[H8tred]

What happened: I had to make a stop by my bank, and seen a guy in an early/mid 80s Nissan hardbody with the hood up. Typical "Farm Use" beater. I decided to be a nice guy and offer a hand. He wanted jumper cables, which I left in the ol' womans car. Didn't have my multi meter either, so I just took his word for it. Took the battery out of my 240 to see if we could get him up and running. Well, nothing. No dummy lights came on, no door chime, not even a spark as I connected the positive. I pulled the main ground and cleaned up all contact points, and made sure there was no breaks in the positive line.

The main relay popped into mind, but in my experience, it doesn't cut all power like that. I couldn't find any bad fuses or shorts. I feel extremely bad that I had to leave him there in that situation. It kept me awake last night, and while I can't help him now, I would still like to know if anyone has a guess as to the cause? Something I overlooked?

[H8tred]

Nobody cares to take a stab? It's not important, but it's one of those things that I would like to have a solution to. For my own comfort.