Father and son were joint owners of the Flat i.e. enjoying 50% ownership.
If both of them were Hindu and have not left any will: have died in estate:
The 50% share of both Father and Son would be equally divided first amongst Class I legal heirs of each of father and son;
Son/Daughter, Widow ,Mother, Son/Daughter of a pre-deceased son (per-deceased means “already Dead”), Son/Daughter of a pre-deceased Daughter ,Widow of a pre-deceased son, Son/Daughter of a pre-deceased son of a pre-deceased son (3 levels), Widow of a pre-deceased son of a predeceased son
So transfer of 100% ownership to wife of deceased son may not be right, if ClassI legal heirs of deceased father are alive.
Your society might have framed rules on deceased member’s flats or there might be provision to frame as per guidelines from Registrar Of Societies…..
The lady may be advised to obtain Letter of Succession/ Heirship Certificate from Court.
Nominee is only a trustee to receive the proceeds of the deceased and divide amongst the legal heir. Nomination does not overrule succession and rights of legal heir.
Valuable advice of learned experts/members is sought.