If Sherlock hadn’t known what it was John was seeing, he would have rolled his eyes; would have cast the spell himself; would have scolded John for freezing up at the sight of his worst fear. They were third year students who had tangled with trolls , aggravated acromantulas, and made friends with the merfolk of the Black Lake. This wasn’t above them by any means, and John was a capable-enough wizard to know how to best a boggart.