Disclaimer - I have not seen the movie.
The chance that this guy drove 300 miles SPECIFICALLY to watch a couple scenes with the Joker involved is ridiculous. That's his own damn fault he traveled such a distance. I have only been to London but I'm willing to place a wager he passed a few theatres closer to his residence while making this trek. Sounds to me that he didn't like the movie, regrets being dumb enough to travel so far to see it, and wants to use this law to make things financially right for him.
But, he does have a bit of a point. The production company can say that those scenes were cut at the last minute, after the trailers had been created. In the US, the few trailers I saw did not include the scenes he described.