Your father-in-law wasn’t in any great deal of danger. Thermometers used elemental mercury, meaning pure mercury that hasn’t reacted with anything. The only real risk from elemental mercury is that it has a low vapor pressure, and can therefore be inhaled, which can be dangerous. Since your FIL was outdoors, the risk of vaporization was high, but risk of inhaling a concentrated amount was low. Had be been doing this in, say, a tool shed, then the danger would have been much greater.
The kind of mercury you hear about that is incredibly toxic and can absorb through your skin are actually mercury salts (ethyl mercury, methyl mercury, etc.) and aren’t as likely to be found.
The kind of mercury you hear about that is incredibly toxic and can absorb through your skin are actually mercury salts (ethyl mercury, methyl mercury, etc.) and aren’t as likely to be found.