Friend lied to get better grades, should I tell?

A friend made up a false story with his best friend so she could get higher marks in his degree, they accepted her stupid story and theyve pretty mucvh guranateed her a high grade, so she doesnt even have to do ell, and they will give her a high grade. I belieev this is wrong, that many students struggle and work so hard, yet she just gets away with it scot free, all because of her lies! I was thinking about anonmysouly sending them a letter and stating this, because at uni, you should not be doing that, your thoughts?