Warming up the car in a STANDSTILL situation is the most dumb thing one can do. It not only harms the environment but also may cause damage to the engine on a longterm run. It is NOT necessary, even if there are temperatures below zero outside.
Just make sure the driver doesn't revv too high when the engine is cold, this isn't good either. As soon as the oil temp gauge is near to 90*C, he's free to go...

Many years ago, warming up cars was necessary but now with modern fuel injection systems, etc., this is NOT necessary, it doesn't make ANY sense at all.

With one exception though: I remember when working in the diplomatic service that one driver of a citizen of a certain country warmed up his engine all the time before driving off from the embassy/diplomatic club.
I asked him why and he told me that this is because of security, the engine of the bulletproof(!) car has to be warm to be able to cope for security risks met on the street. This makes sense to me, a cold engine may not be a good thing to have when you have to escape from a dangerous situation. Just wanted to mention this, just in case...