As one that has experienced suicidial ideation for many years I can tell you for certain that there is nothing you can do. From 15 to 33 I obsessed on suicide.
I have that mostly beat now but I have had a recent return of these desires. While I did want to stop hurting, I did not tell my therapist until after I had gotten past it. The reason was I didn't want anyone to stop me. In my last episode, I even stopped taking all my meds knowing that this would make me extremely suicidial. And oh boy did it do it. But as you can see, I'm still here.

Nothing but fear itself could stop me. No person could. I've learned through the years who not to talk to about it, while it's going on. Primarily anyone that knows where I am.
I'm really sorry this happened to you. I've talked to my therapist about this and she said to me there are only therapist have not had a client commit suicide yet. With your speaches and this web site you're reaching far more people than a "normal" therapist would, thus you've increased you're chances to run into this I think it comes with the territory.
I wish I had the words to make the self doubts and second guessing yourself that any of us would likely be feeling after something like this. You were in his life very late, you were a tiny part of it and I'm certain that it was only a matter of time for this man. This would have happened no matter what you did.