If he's still on the visa sticker then he can resign and that sticker will remain valid until the 90 days have lapsed.
If he's on an extension of stay based on employment then it will be cancelled the day his employment finishes, as well as any work permit he has. He will then need to leave Thailand the same day his employment ends.
if in Bangkok you either go to Muang Thong Thani or Chaeng Wattana dependent upon what visa or exemption you arrived on. Once you know which it is then search the group to see which office you go to. And no, you didn't arrive on a Thai Pass.