I don't think you would have a case. The regs are a little vague. They say it is illegal to have a snakehead in your possession and it is illegal to transport them anywhere. But then the FWL website says they are good tablefare. Not sure how you would know they are good tablefare if you can't have one in possession, yet the regulations say there is no daily limit on nonnative fish which the snakehead is. Kind of confusing right?
My suggestion, leave the guy alone. The snakehead is here to stay and him releasing them back in the water he caught them in isn't going to change that fact. So why screw with the guy especially when the regs don't say he has to toss them on the bank. Until FL has a clear regulation in place you're trying to start a fight for no good reason.