By becoming true friend and pointing out what is not good. It is enough that we don't cheat because if we don't cheat, those friend who do cheat will learn from our example. It is not easy to stop others from cheating.  All you can do is stop cheating yourself and pray to the Lord. If a close friend seeks your advice, you might just mention that it is not good. It is not that easy to prevent a man from cheating. It is very difficult. It is greed that drives him to cheat. You just pray. You behave properly. You do your best to become a good example.

--Swami Radhanandaji