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The guy in that video used CO gas at his attempt. Suffocating extinguishing makes no sense. Instead sufficient water has to be used, to cool down the film.
Likely solid CO, as CO-snow, could have worked too.
RaushcenOderKorn
NO, you cannot extinguish a nitrate fire, only contain it until it burns out; It generates its own oxygen and will burn under any liquid or in any inert gas environment.
In fact, the WORST thing you can do it to try to smother the flames, as that causes incomplete combustion and results in even more unburned, toxic and combustible gasses which accumulate into a pocket and cause a "flashover" and explosion.
The best thing to do is to determine if it is even worth keeping, and if it is, to store it properly. That being said, almost no one outside of a government institution has the proper facilities to store nitrate film safely.
Our nitrate vaults have a fire suppression system that isolates each reel of nitrate in it's own cubby hole. Should a single reel combust, it can burn itself out while the fire suppression system cascades a sheet of water around the burning reel. While it won't extinguish the film, it will allow it to cleanly burn-up while keeping the fire from spreading to adjacent reels.
RaushcenOderKorn has the right idea in contacting the Bundesfilmarchiv. They know the regulations inside and out, plus they will be able to help the owner evaluate the historical value of the film. Often we have collectors who are simply eager to dispose of film that turns out to be very historically important.