I've unfortunately witnessed suicides and attempts close enough to tell you that it is unlikely you could have done anything as a non-related person far away.

I've unfortunately witnessed suicides and attempts close enough to tell you that it is unlikely you could have done anything as a non-related person far away. Taking him to lunch and treating him as a human being would probably been a super nice thing to do for him. I know you feel that the part where you think "man I should have said something different" makes you feel bad, but people do much, much more regrettable gaffes they realize only after the fact. Like, for instance, a spouse who considers maybe to lock away the firearms, but not doing it (an unfortunate example from real life, not mine luckily). Luckily most people are not experts in suicide. Even though if you had concrete intuition that something was off, having no experience of this before, you probably could not tell if the intuition was pointing at something actionable, or imagined. Persons who are under treatment, with their family, still end up performing a suicide. I know it feels bad for you, having interacted with him briefly before his passing away, but you should not feel guilt (even though its a natural human reaction you do).