There are three things that affect strategy. You and your opponents/teammates, the setup, and the dice.
Because there is no affect of dice G1 for instance, the only factors are you and the setup. This comes down to the fact that the setup offers some moves and you pick the best one.
They were not intentionally cheating, I am sure, however, they are still breaking the rules and will have to be penalized accordingly. Which probably means returning their ships to the proper sea zones, removing the infantry and surrendering whatever collected money they had to the bank.