Nikah is an open declaration of a contract by a man and a woman to live permanently as husband and wife. Therefore, the main objective is for the society (in general) to be aware of this fact that this couple has decided to live together as husband and wife. Including one or two close relatives as witnesses and conducting this ceremony secretly do not fulfil the requirements of Nikah and hence, Nikah would not be valid. For Nikah to be valid, it needs to be announced either through societal channels or through legal channels. In addition to this, all the legal requirements of a country where one is residing related to marriage registration must also be fulfilled.
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Discussion 45473
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In the scenario asked in the question, the requirements seem to be fulfilled (both societal and legal). Just because some of the friends were not aware of it, it does not render the Nikah void.