Just wondering if I should tell him and show the proof, or just let it be.
Most likely you'll get an arguement and told you don't know squat, especially if they bought it from someone else, thinking it was genuine and now realize they were duped.
If you're no longer interested I'd just walk away, maybe just say there were discrepancies. If they really want to know what they are, they'll ask. If you're still interested definitely use it as a bargaining tool and walk if things get heated.
If you have any info like VIN or trim tag the mods here or at TC site may want to add to their databases, so any future buyers know that at least the paperwork is fake, if not more.