Will surely be a fine as I presume ref saw him throw it and could've sent him off if that was in the rulebook? It's stupid no doubt but club would argue flares shouldn't have been on the pitch in first place.
If he was booked then they would be less likely to do something. You may presume the ref saw it but he will tow the party line and say he didn’t if they instruct him too. I get that he was happy and was a spare of the moment thing , but I am fully expecting a ban for this I fortunately.