He will have to ensure the following:
- skin on
- 40cm + in length
- don't exceed the bag limit.
Frankly, if he doesn't observe these rules, he'll get done. He may argue his case, but if he gets away with it, everyone will be filleting fish, saying they bought them, and producing receipts. Fisheries will be stuffed, ain't going to happen, he will not win.
Remember bag limits are "in possession", so if you had, say 3 Snapper fillets for bait, and 4 snapper on board, you would be over the limit (if one person aboard).
I think your Bro needs to change his practices, unfortunately.
Tim