1) Start a private server running Escape from Stratis. Have all the roles be AI except for one slot (which is empty). Have the players take over AI roles and start the game.
2) Start playing, and once in combat, have one of the players force disconnect (ideally by ending Arma 3 in the task manager or something similarly drastic.
3) Have the disconnected player rejoin the server while the mission is still running, and get him to try and play his original role (which has now been taken over by an AI).
4) The player should be unable to continue the mission until he switches to a role that has no occupant, human or AI.