Grounded kill wire is what I was thinking as well. There's a spade terminal on the one I had, mounted right near the coil. In operation, you would ground that to kill the engine. If there's something wired in to that terminal on your buddy's engine, have him check for spark after unplugging it.

We're pretty sure this is a 42? There's a nearly identical 35 that uses a set of points under the flywheel. Wire leading to them from the outside is a dead give away....