It depends on a lot of things. In the OP's example, I would likely politely let them know the river is closed. Probably low risk that the dad would react aggressively. On a trip out west once, I drove an hour to a remote stream to fish for wild, native Apache trout. Very remote place, but when I arrived, there was a young couple (probably early twenties), fishing bait. Clearly illegal. But I thought, I'm planning to hike far from my rental car. If they don't receive my message well, I could be stranded (they could slash my tires or worse), with no way to get help. My wife knew exactly where I was, and would have sent help if she didn't hear from me by late afternoon, but that would have been a really lousy way to end the day for both of us. I could have said something and left, or just left, and called it in once I had cell service, but there goes my one chance to fish for Apaches on the trip, which wasn't likely to be repeated any time soon (if ever). So, I said hello, and fished my way upstream of where they were. I did call it in on my way back to the hotel. Sometimes, you have to exercise good judgment with respect to doing the right thing.
