No, because it's terrible. There's no need to break encryption to allow you to report a user. You'd just report via a copy of an excerpt of the conversation and leave the rest of your communication private. If the user can't tamper with the extraction of that excerpt, you can trust it is correct. You could even extract hashes from both the reporting party and the reported party and compare them with zero knowledge of the actual conversation.

More sophisticated HNers can chime in with zero-knowledge proofs and whatnot to show that their argument is DOA.