Divorce in spcl marriage act possible w/o having registered marriage?

We doubt about the validity of your nephew marriage. All inter religion marriage such as between Hindu and Christians have to be solemnized and registered only under the Special Marriage Act,1954. The marriage in Arya Samaj Mandir only has legal validity when marriage is solemnised between two Hindus only and not in cases when one is Hindu and another is Christian.The Hindu marriage under the Hindu Marriage Act,1955 requires both parties to be Hindus. In your case you nephew's wife was Christian and if before the Hindu marriage she never converted to Hindu religion, her marriage with a Hindu will be Null & Void.

In addition to this, there are various judicial pronouncement wherein the Court has observed that Hindu married to a non-Hindu in accordance with Hindu rituals cannot seek divorce under the Hindu Marriage Act. 

Further, you have already mentioned that your nephew has not registered under Special marriage Act, so it is clear that he will not be governed by Special marriage Act.

Furthermore, we will have to see all the documents relating to your nephew marriage before giving you the correct legal advice.

Hence we will first examine the validity of the marriage because if your nephew's marriage itself is not valid, the question of divorce does not arise.