And you might have taken a different route home that day and never witnessed the event, much less interacted with them.

It's been my experience that they who have succeeded didn't let a day-one failure stop them. They sought help or figured it out.
The pair either did or did not become lifelong enthusiasts whether or not you helped. You may have sped things up, though.

I understand how you feel, but don't beat yourself up too much. We all have regrets.
Look at it this way: You've grown.