Seems to me that common sense would dictate that you NOT allow it in the cabin. But then we all know the rules are sometimes not so common sense. Interesting it would get that far in the first place.
I'd grab the gate agent and the Captain to let them know. Surely the Captain wouldn't want it in the cabin. I'm almost positive the petrol would be taken away right at the a/c door. I've had Giant scissors removed from a pax and put in an envelop in the flight deck for safe keeping. How they made it past security in the first place, I have no idea.
If anyone has a better answer, please let me know.