A division bench of Delhi High Court held as follows,

From Paras 11 and 12,

11. In our view, even though the conclusion reached by the Family Court Judge on this score is correct, i.e., that the divorce petition cannot be rejected in part, arraying a third party to a divorce petition is neither proper nor necessary. A necessary party is one in whose absence no effective decree can be passed, whereas, a proper party enables complete and final adjudication of issues involved in a given lis.
11.1 The alleged adulterer is, to our minds, not a necessary party as a decree can be passed in his/her absence. Likewise, the adulterer is not a proper party since the issue concerning adultery can be adjudicated without making the adulterer a party to the cause. Proof of adultery need not be conflated with who should be arrayed as a party to a divorce action.
11.2 A divorce action is a lis centered around the couple who have entered into matrimony. A third party [who does not claim the status of a spouse]
has no locus to intervene or seek impleadment in such a cause. [Also see Manjul Joshi v. Bhavna Khurana, 2024: DHC:4170-DB].
12. The alleged adulterer (third party) can either be summoned as a witness or other evidence can be placed before the Family Court to prove adultery. Therefore, on this count, we are not in agreement with the counsel for the appellant/wife.

Index of Divorce judgment is here.