1) Co-defendant 2) Co-respondent 
A term sometimes applied to a third person, who is accused of committing unlawful acts with the defendant, by the party seeking a divorce from the defendant.
[n] - the codefendant charged with adultery with the estranged spouse in a divorce proceeding
co-respondent noun the codefendant charged with adultery with the estranged spouse in a divorce proceeding
a joint defendant, charged along with the respondent, esp. a person charged with adultery in a divorce proceeding.