Are you sure he was waiting by the correct gate area?

I'm sure if we was there at 11:50 am he would of seen Frontier gate staff, other passengers boarding, and heard announcements in the gate area. Typically, the frontier gate staff returns to the podium computer for a few minutes to close out the flight - so there of been an employee at the gate until that 12:25 pm time (maybe minus a few minutes for them to push back the jetbridge).