I seem to remember that StarCraft 2 shares out the quitting player’s resources to the team, and maybe even the income from resources currently being mined. That could work here. Plus control of the units and structures. You can continue to produce units/workers from their facilities which you will have control of. Not only that, you also get access to all their units and buildings, so you could build different civ units. It’s actually a real advantage if your ally quits sometimes.