That's a tough one. I think the above posters have given some valuable insight. My gut reaction was to let it go, but I think my younger self would have had a lot of trouble doing that as I tend to define myself by principles . . . and even at my age I would still have trouble letting it go but not as much. But I think that the main thing one needs to do is focus on themselves and what makes him or her a better and happier person. What they are doing is not right but relatively small in the big scheme of things. If you think there would be any negative consequences for you I would for sure let it go.