"Don't be a tattle- tale" (In Australia: don't be a dobber)
Why do we say this to children, yet we expect adults to be whistleblowers and report corrupt or illegal practices? On the contrary, people who are willing to report to appropriate authorities the mis-behaviour of their peers are a vital part of the rule of law.
"It takes two to fight"
When a bully is involved, it often only takes one
"Two can live as cheaply as one"
"You can't have your cake and eat it"
The only way to have your cake IS to eat it