Friend threw mercury in a fire, is this dangerous?
Note that I generally do not use a lot of boldface in posts. Please recognise that it is required here.

Immediately get out of the room! Keep children and pregnant women away! Ventilate that room very well for a long amount of time! And never let your friend handle a mercury thermometer again!

Liquid mercury is not as dangerous as some people make it to be; a number of mercury compounds (e.g. dimethyl mercury) are much more toxic. However, that is because of the way most people would be exposed to mercury typically. If the thermometer breaks, there is a puddle of mercury on the floor and barring yourself doing anything stupid it will take a while for the mercury to fully evaporate. It’s inhaling mercury vapour that is most dangerous with respect to liquid mercury.

Mercury’s boiling point, according to Wikipedia, is some $350~\mathrm{^\circ C}$. A fire in a fireplace is typically hotter. Thus, by pouring mercury into the fire, your friend has chosen the most effective way to generate mercury vapour quickly from liquid mercury. Unless the fireplace is a closed oven, this vapour will now gladly move around the room with air currents, thus being easily breathed in and absorbed by the body.

Of all the ways to handle mercury, your friend chose the most dangerous and the most stupid!