There's a been a few similar instances in my life that have led me take up the personal practice of "Always say hi or wave to friend when the chance comes around, because there may not be a next time". It came about because I tend to see a lot of close friends and looser acquaintances on a day to day basis physically in the world, and there used to be more times than not where I wouldn't bother crossing the street or stopping for a minute to chat. Later I realized this costs me almost nothing, and even for less-close relationships, I'd prefer to have put in the tiny amount of effort to walk up and show them they're worth even that much before they overdosed or moved away or committed suicide. It's not always opportune, but what else is life for?

Granted, in retrospect, there's not really ever a sufficient amount of interaction you could have had, but if I see someone inside a cafe that I'm walking past, it's worth popping in and at least saying hi or waving from outside.