First of all, NOT in a public comment.
Second, I think it depends on how well you know him or her. You might start with a PM, thanking him/her for the comment and talking about it, maybe asking more about what he'd liked about the story. Maybe go back-and-forth via PM several times, before asking something like "How would you feel if you find out someone on a site like this was someone you knew?", kind if ease in to it, just in case the person as less of a perv than you or I.
Does that help any, Kim?