If a game desyncs you should quit and make a new game, thats why this window keeps popping up. A desync means you and the other players are not playing with the same set of data anymore. EG in one version of the game player A died, in another version he didnt. There is no way to recover the game  as far as I know, even removing or replacing the player with a desync isnt going to solve the problem. There is only one choice: host a new game.

Several reasons can lead to desyncs, three come to mind at once:

- the installation of one player is corrupted

- he is using a sim mod and forgot to disable it before logging on

- the data sent from the player was corrupted

So if he didnt use a mod and the game installation is not broken it's most likely a data error. The player who desynced can try to reinstall the game but I doubt that is the problem.

To tell the truth, I can't even begin to narrow it down if I don't have more information on that specific players installation.The best I can do atm is to give you some general advice:

- If it happened only once, it's most likely a hick-up.

- If it happens in all his online matches I'd say it's a sim mod.

- If he has problems with single player too and his game crashes form time to time or shows unexpected behavior it might be a corrupted installation. I more guessing here, there could be a bunch of other reasons too.

If it keeps happening, ask him to uninstall, delete the remaining files and folders of the installation (in My Documents AND the main game folder!), use a registry cleaner (make a backup first!!!) and ask him run a clean installation.

Should the problem still exist after that, please make a DxDiag (tutorial is here) and contact Stardock tech support.

Good luck, I hope you can sort it out!