This was a very sad story. Charcoal fires are prodigious producers of CO. That said, I'm sure we've all seen photos of arctic explorers or Himalayan climbers melting snow in their tent with a small stove. I suppose the general answer is that it is always dangerous, but sometimes you can get away with it. Some specific fuels and stove types produce less CO per unit heat.

http://www.backpackinglight.com/ has several scientific reports (some premium$) with specifics.

For me personally, a candle lantern is about as far as I'd go, and even "cooking in the vestibule" I'd want a good breeze.