...is to drop the client. Seriously.
He wants Orwellian monitoring over his network that is not only unfeasible but would eventually prove completely ineffective. If he's this paranoid, what's going to happen when your kludge of a system inevitably misses a message or two and he decides that caused someone to fall victim to a scam? He's going to come after you with some shark lawyer and make your life incredibly annoying, that's what. In the end, his idea will not prevent scams and the like. It's only going to further a "big brother knows best and sees all" mentality. On top of that, it shows a frightening lack of trust in his family - both in their ability to "do the right thing" and in their general intelligence. Your best solution is to drop the client and not feed his totalitarian ego.
On the other hand, if this is really you wanting such a solution, the trust issues apply even moreso. Learn to EDUCATE instead of spy. You will have much better results.
And finally, if you're an ISP too clueless to do something on your own, GTFO Slashdot with your asking us how to spy on your customers. You should be ashamed of yourself.
tl;dr - Your plan is a bad idea all around...