In the grown up section, instead of saying "obvious liar", we'd say he's mistaken or incorrect. Instead of personal attacks, just discuss the facts as stated.
And I believe a bunch of us already called out the impossibility of what I think you're talking about. That is the correct way to address that in the grown up forum. If there are repeated just impossible stuff than I'd entertain a trolling charge, but I do not think that is what is going on here. People who made it to 400 pounds usually WILDLY underestimate how much they are eating so it makes sense to me that he'd THINK he was eating that, even if he's not.
People can be wrong without it being malicious/lying. Lets default to that here.