I think it has to do with whether or not you are talking about someone you don't know but was kind enough to wait or someone you ride with all the time. If a Stranger, then I would not ride by, I would ride up to and engage in conversation and either drop back or then ride ahead. If a bud, usually whoever is waiting starts off as I ride up so I can draft or if stopped then I would ride by, signaling OK I'm here let's get riding. It would probably be impolite to not acknowledge that the person waited. Always give them a thumbs up or thank you.