You are not the exception to the rule. You will not cure them of cheating, because you're an awesome person. A cheater will always be a cheater. Especially, when they are cheating long into adult years.
Sometimes you put walls up not to keep people out, but to see who cares enough to break them down. - Socrates quote pp Brief Thoughts On Friendship and Betrayal - Saving More Than Me I can see this in some students.