Friend cant join back hamachi game?

So my friend and I are playing over LAN Hamachi since I installed it on his computer while I was at his house. I saved the MP game now we can't resume after he left the game and I continued playing because it says that the game already started and it removed him from the players list. How can I fix that?

If he tries joining the lobby when the save is loaded from after he left it says "game already started"