Now that they've 'kept' his camera what actual proof do they have that it was your friend that took the recording? They dont. If they wanted to prosecute they should have done it immediately, reporting him to the police then and there. Because they didnt they dont have a leg to stand on.
They camera may be your friends but they would never be able to prove it was they who made the recording.
Tell him to get his camera back immediately, otherwise 'do' them for theft. They dont have any right what so ever to confiscate someone else's property.
Put it this way, if your friend demands his camera back and the cinema call in the police the first question they'll be asked is why the incident wasn't reported at the time. Do they have an answer to that??? I doubt it.
I'd put money on who ever caught your friend has kept the camera for himself and destroyed the copy of the film (if it was found in his posession he'd get done!).