Have them check with the airline to see if they do need one or if a 20mo old can sit by themselves. That said, personally I would bring a car seat for a 20 month old. When/if they get tired it will make it easier for the little one to fall asleep and rest. Trying to control a 20mo old can be like herding a cat and an aircraft could make it worse. For piece of mind and parents sanity I'd tell them to bring one.
Also if they do, they need to make sure their car seat is aircraft approved. If it is there will be a sticker on it saying something to the effect of "This restraint is certified for use in motor vehicles and aircraft" or even FAA approved.