WARHAMMER 2 coop campaign wrong loading?

Playing wh2 with a mate in coop, his Wi-Fi sucks and sometimes he has lag spikes that cause our save to corrupt? I say that because when we join we get random factions instead of the one we previously had and can’t open the game at all. Lost a 27 turn Skaven + pirate aliance and we’re pretty heartbroken about it. Anyone in here know how to fix this?