Some anti-depressants trigger suicidality in people, it is well known, printed as a warning, but seems to be buried in discussions. Going cold turkey off your meds can also mess you up.
I'm sorry about your friend... you did what you thought was right for him at the time, and who knows what really happened... but I do wish more people knew things can go horribly wrong in therapy. People treat it like it cannot cause harm ("can't hurt, right?") but it absolute can and does.