Once your friend's wife resigned from the job, there was no reason for HR of that company to obtain resignation of your friend under duress. It appears that HR wanted to remove your friend and they have used their marriage as an alibi.
You have written that "After few days Regional HR, RSM, ASM & ACM again pressurized my friend and asked him to update his resignation or else company will terminate him." When authorities from different department join together to get the resignation of your friend, it shows that your friend would have been removed from the job anyway. That his wife also worked in the same company was just a ploy. Yes, suppression of material facts is a misconduct but that does not merit removal from the job!
Whether the forced resignation was legally challengeable or not that needs to be seen. Please confirm his designation, how many persons reported to him, whether this clause of husband and wife cannot work together was inserted in the appointment letter or not, whether the company has approved and certified standing orders or not etc.
5th April 2018 From India, Bangalore