I may be wrong but we didn't see the owner of the diner get locked in the fridge. At the beginning he wedges the door open. The next scene saw the police & CSIs turn up and discover the murders. Presumably the owner locked himself in the fridge after he'd committed the murders.
We didn't see him actually being locked in the the fridge/freezer. Ad Dodger says - after killings finished he went back in and used mobile phone to call for help.