Colleague erroneously got a grade that is too high I know that for a certain project a colleague with whom I have worked on the project erroneously got a grade that is higher than he should have gotten (the administration made a mistake is my guess). He told me about this himself, but also told me that he is not planning on changing it. The course is a 12 ects course, so has quite an impact on someone's gpa.

Now I have two thoughts:


It is not fair to 'betray' a colleague who trusted me with this story
I also worked on this project and I got the correct grade, it feels unfair. And I also simply feel that it is simply not just in a way.


What do you think? And should I take action or not?
Answering from an ethical perspective. The grade was entered in error, if the other student was unaware of a mistake they would have been morally fine, it is not their job to double check the grader's work. 

But since they are aware of the mistake it is their obligation to inform the grader. By intentionally keeping that information from the grader they are deliberately breaking ethical rules. In addition, by telling you about it they are compounding the original wrong by making you a party to their original transgression. 

Unfortunately your classmate has made you responsible for their behavior because now you have to choose between doing the right thing (reporting them since they will not step forward) or being silent. 

I can't tell you the consequences of that choice, but possible consequences of turning them in are social rejection and possible consequences of staying silent include academic sanctions against you if it comes out that you knew and did nothing. Just remember, they chose to put you in this mess, and for that you owe them nothing.