When you’re playing a team game and your teammates resign (for whatever reason), you should get a slice of or all of their resources and units.
There’s been times where I’ve been stuck defending against two to three armies while all my partners military and resources are just sitting around.
In my suggestion, If they resign, they aren’t just leaving you out to dry but giving you tools to stand a chance still. In terms of whether that is fair for the other team, they would have been fighting against said resources had the individual not resigned anyway.