I'm surprised it actually worked. Liquid mercury isn't really that harmful, it's the vapors that get you. I'd be concerned about it affecting me too, since I'd also be sitting at the board.
Comments above say the opponent was hospitalized but not killed. I can’t swear to the veracity. There are many forms of mercury though and they vary in toxicity.
I’m no chemist but understand the liquid mercury is safe to handle for short periods provided you wash your hands well. It can absorb through your skin over time though.