he's scared that anyone official, like his gp
GPs are bound by the normal confidentiality rules. So nobody is going to 'tell' Your mate would have to own up though at the next medical.
More importantly though is that everyone self certifies in effeect before they commit to flying. The 'I'm safe' acronym springs to mind.
Active component in St John's Wort is akin to Prozac and any of the other SSRI type antidepressants.
As humans we all have ups and downs, if you are properly depressed then you need professional help, not the best intentions of someone in a health shop.