I'm generally pretty astounded by how many people end up killing themselves or others this way every year. It's a fire, don't stick it inside your highly flammable tent. Fires and things in which you've had a fire should always be left outside as they'll still be chucking out gases, and with charcoal fires they can still be burning somewhere without much noticeable heat. I spend a lot of time camping with all manner of fires, stoves etc (historical re-enactor) and I've never once thought "I should bring the fire box inside the tent so it doesn't get wet". Keep your burning materials in the dry, and the box/BBQ/whatever outside.
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.
I guess the problem is ventilation. Warm air always goes up, and I guess arctic explorers have a sort of an opening at the top of the tents, like a chimney, to let the gas out. Otherwise they'd die just as much (also, in subzero temps the definition of "warm" is different than when out camping - there I guess charcoal gases could hover, especially in a sealed tent where it's warmer than outside...)
