My brother and his first wife both were members of a karate club.  It turned out that his wife was having an affair with five (!) different guys from the karate club.  My brother's "friends" who were members at the karate club knew about these affairs, but they didn't tell my brother about it.  Anyway, my brother's wife finally put a few pictures of herself naked with these guys from the club in different places where my brother would find them.  I guess that was her way of confessing.

     When my grandmother heard about this, she told me,

"I knew karate did bad things to nice girls!"

A few years later, my grandmother's sister and I were talking about Jimmy Swaggart, a Christian evangelist who kept getting caught with prostitutes.

     "It's sad, isn't it?" she said to me.  "It's sad how men have such difficulty controlling that part of their lives!"

     "Oh," I said.  "Is that why my brother's wife had an affair with five different guys at the karate club?"

     "Well," my great aunt replied.  "She was demon-possessed!"

The word you want is "cheater", not "traitor".

Your friend needs you to tell her the truth. Whether you conceal it to save yourself a little pain is entirely up to you.