Should I Keep My Friend’s Sin Secret?

Your friend comes to you with a secret sin. You are his accountability partner, and he asks you to keep his sin secret. How would you respond to him? Is it okay to pledge confidentiality or have an expectation of it? Does confidentiality have value in the context of confession, or does it interfere with the benefits of confession? Are there instances where we should tell others about a transgression, even if the guilty party is against it? What biblical principles should guide us regarding sin, secret keeping, and sharing inside of a biblical community?