Friends don't 'allow' others to cheat or behave wrongly. They can make a suggestion but it is not their place to bar someone from doing so. A friend who observes another's questionable behaviour may inform on them but they have no obligations to do so. The mark of the friendship is having the freedom to voice an opinion. If the transgressor continues with the unfortunate act, then it is up to the friend to evaluate the basis of the friend relationship and decide whether the action is forgivable or not within their personal limits. Basically, it is up to someone whether or not to put up with someone else's behaviour. Now, if this was putting someone in harm's way, there always is an authority to turn to.