I'm not sure if they really gained a massive advantage (depends on if they would have had enough attackers to take Gao and gift it to Dantski and then attack the cannon-stack).
Though I agree that they should have either waited for you and discussed it or reload the turn (without you ordering them to do so).
Though I agree that they should have either waited for you and discussed it or reload the turn (without you ordering them to do so).