Indeed you can fake anything on a web browser with local javascript (e.g. plugin). He would need to give his FB password to a trusted third-party and have him login in and check, OR he would have to log into a trusted third-party computer (through VNC or whatever) and access his FB through there (the computer admin would be recording the desktop session).

The issue is one of trust. You cannot trust someone who speaks on YT, nor can you trust a computer you don't control. You could trust code, but you can't look at the code running on the guy's computer. You could however trust a third-party, who would have to be involved in the verification. That's the only way IMO without FB involvement (which would anyway also be a third-party).

Faking this would be a good way to publicize his t-shirts. But the value of his shirts come from the funny catch phrases he came up with (which you can copy and easily make/order your own shirts with). Plus he found/published that museum kill room stuff. So my two cents is he's telling the truth. If he's a shill, he's not good at his job.