I re-inserted the rows into xf_conversation_user from a backup, using their old userid.
I updated the userid from the old to the new for the owner_user_id and the last_message_user_id.
I rebuilt the conversation cache (and several others)
Still no conversations for this user! Other user's conversations with this individual seem intact but the "conversation participants" list shows "Unknown User".
If anyone can provide some insight into this matter, it would be much appreciated. Are there other tables that require manipulation?