Sorry, I forgot you hadn't yet watched it - well, at least you can do so eagle-eyed and report back if you see any chicanery!
But he was smart and the lemon trick was cleverly done however it was achieved.
I think that is less of a con than the substitute dog - had the audience/ viewers been aware of more than one dog before they voted, that would have been a different story.
Anyway... time to take out the dog.....enjoy your viewing of BGT ( I expect you to have solved the mystery by the time I get back!)