Having a bad day...
I assume you are familiar with the DHC-6 Twin Otter, or 'Twotter' as it is usually known.
If the thermometer fell out of its mounting hole then I think you could very easily just take it along to hold in your hand and stick out the slightly-opened pilot's sliding window if you really needed to know the OAT exactly. Okay, apply CRM and have the PNF do that!
Given that this was the Sahara Bloody Desert then it is pretty safe to assume that one could look at the OAT on the ground, apply the standard lapse rate of 2 degrees C. per thousand feet and know the freezing level if one was going to fly in IMC.
Or, if you were cheesed off about bits falling off your Twotter then you could look in the MEL, find that a missing thermometer was a no-go item and ferry it back to base just to prove a point, what seems to have happened in this case. So management took the point, in a market with plenty of other pilots on the market, and fired this guy!
Is there some part of this that is difficult to understand?