If i have to guess the only way you might get banned is if you download a crack of the game. (a free full copy of the game you found on a website)
Apart from that mods or cheat codes are all fine and fair in SP . For some campaigns you might need some cheats or mods to achieve your goals and interesting playstyles or simply being able of having more mechanics.
If your friend downloaded a crack of the game just don't do the same. You can find games for dirty cheap on some websites and overall is just wrong to do such things. People work on games and if you are a worker, creator, supplier you know damn well that you should get paid for your service