You don't. Its none of your business. If it wasn't your ex would you care?

Get a new friend and leave them to it.

It depends do you still had feelings for him when this happened and she knew? The you should confront her, asking why she did it BUT If you didn't had feelings for him ,well forget it is their life

Was the ex an ex when the friend slept with him/her?

If it was after you broke up, you let it go and move on.

If they cheated on you, let them go and move on.

Holding on to negative emotions like hurt, anger and betrayal only cause you pain and confusion. Easiest way to deal with that confusion and heal the hurt is to let it go and involve yourself in things that help you feel good.