My two older kids competed in climbing for several years, and the local competitions were often on an honor basis, with each competitor filling out their own card as to which routes they were able to complete during a specific amount of time (called a red point for those who care). One girl on the team always came in second to my daughter, and the kids were very aware once they began the season in earnest with judged competitions that this girl had to have cheated, because she could no longer perform at that level. She would claim mild injuries, etc. to explain away the differences.
My daughter, the team captain, kept quiet for a long time, but it ate at the entire team. She finally decided that it wasn't fair to anyone to not address the issue, even if there was not real proof that that girl had cheated.
Instead, during a team meeting, she brought up the need of each competitor bringing their best effort to the group and pointed out that when one of the team slacked off and made excuses, they weren't being honest with themselves or the team. She made eye contact several times with the girl in question, but my daughter never named her specifically.
The next season, this young girl filled out her card accurately, and the dynamic with the team was quite different.
Is there a way to approach your issue in a similar way since there is no hard evidence of cheating? Good luck - it's a tough call, but I do think addressing the issue even in broad terms will be better than saying nothing at all.
