Your friend is calling for trouble. Half knowledge is always bad and this what he has of legal provisions. Hindu Marriage Act, 1955 which applies to all Hindus domiciled in India, section 5 which provides for Conditions for a Hindu Marriage wherein prohibition of having a living spouse at the time of marriage (i), as well, prohibition of marriage between parties who are within the degree of prohibited relationship (iv) or are not sapinda of each other exists(v) , along with this, there is a proviso attached in clause (iv) and (v), which is "unless the custom or usage governing each of them permits of a marriage between the two".
What it means if the custom or usage governing them permits that their marriage will not be null and void under section 11 of the HMA. More so the section 11 clearly states Void Marriages - Any marriage solemnized after the commencement of this Act shall be null and void and may, on a petition presented by either party thereto against the other party, be so declared by a decree of nullity if it contravenes any one of the conditions specified in clauses (i), (iv), (v) of section 5, those I mentioned above. What is to be noted that "solemnization of marriage".
I bigamy prosecution what has to be proved by the prosecution, the solemnization of the first marriage of the accused as well the solemnization of subsequent second marriage during the existence of first marriage.
The argument your friend gives about null and void marriage if he marries cousin as that will be within prohibited relationship is same as null and void marriage during the existing marriage of the accused of bigamy criminal charge.
The Supreme Court has laid down very clear principle on this, the validity of second marriage is not being questioned (which otherwise is no doubt null and void) while prosecuting the accused of bigamy charges, the very fact he was having a existing marriage and still he got married second time, if both his marriages are completely solemnized according to customary rites and ceremonies of either party thereto and these customary rites and ceremonies for both his marriages are proved by sufficient admissible evidence by the prosecution than the charge of bigamy will be proved that make him liable for punishment under section 494, 495 IPC.
Tell your friend we the lawyers who have put our lifetime in the profession are not fools; vague talks of legal provisions like this is just like taking medicine without doctor’s advice.