It is the kill wire, or the engine wouldn't stop when you earth it - plus it is the kill wire under the Tecumseh wire colour code. The most likely reason the engine won't start when you attach the kill wire to the mower is that one of the safety switches is grounding the wire on the mower. Were you sitting on the seat? Did you have the cutter clutch disengaged? Unless your answer is yes to both, the kill wire was grounded when you tried to start the engine.
The square voltage rectifier will be a bridge rectifier, consisting of four diodes. Where did he put the bridge? Is it in the mower's red wire, which is now connected to the engine's yellow wire? What he has done may be a perfectly good solution, but at this point I don't know what he's done.
