Dennis M. Dennis offers these suggestions to deal with a backbiting colleague:
Delay any response. Give yourself time to think clearly.
Reflect on the situation. Ask yourself what you want to accomplish by meeting with the offender.
If you decide a meeting is necessary, have face-to-face contact whenever possible.
Express to the offender how his behavior affects you. "I'm hurt that you talked behind my back about ... " or "I'm disappointed that you said ... "
Listen to the other person's perspective.
Talk about creative ways to keep it from happening again. For example: "Next time you have a problem with me, come to me first."