DISCUSSIONS on “Rules for online games in HoMM3 and formulas for calculating player rating.”

that he did not change the turn?

"Usually" people get disconnected if there is a chain on a Griffin Conservatory, Dragon Fly Hive, or Pandora's Box.

A suspicion arose — load the save after the game and check. If there really was a chain on a Pandora's Box (unchecked Griffin Conservatory, etc.), and in the next day's save it is not taken or not even checked, then you can raise the alarm. Or if there was a suspicious turn that does not align with the built chains.