There’s certainly a good argument for Neal and August having had enough in common to have been friends. I’m just not convinced that August spent much time, if any at all, in America, after splitting to Phuket. If not for that line of Regina’s that he looked familiar, I wouldn’t question it at all, and would think that he didn’t set foot on American soil from the time he left with Emma’s money, until the time he came back coz his leg started hurting.
If so, then the only time he could have met Neal in NYC was when he came back and was on his way to SB.
But if that wasn’t just a throwaway line from Regina, then there could have been another time in the past that August and Neal could have met up in NYC.
Unless her line was just meant to drop the hint that August was someone from FTL, in which case, we’re back to him potentially not having been in America since before Henry was born.
For now, I’m betting on this mystery scene being in ep18 and showing August checking in on Neal when he was on his way to SB. Making sure he knew where he was. Confirming the deal about sending a postcard. Giving him the heads up that the time was coming.