Serpentine belt

That sounds like the compressor is locked up, either it was slugged with 134A Liquids do not compress, if the system was charged improperly and liquid 134A got to the compressor it will stop it immediately and can destroy the compressor.

However, it sounds like you are saying that the belt comes off regardless of the AC being on or off? If that is the case it is likely that the clutch bearing on the compressor has failed and the pulley cannot rotate freely. With the engine off and the belt off, reach in and see if you can easily turn the AC pulley. Next you would need to check the belt, once it came off the first time it may have broken the reinforcing inside it and need to be replaced. The other item would be that the belt tension-er is not pulling the belt tight enough, for that you replace the tension-er.