Yes, the mother's share in the predeceased son's could be disposed at her discretion. Since the property is Tamil Nadu, her share in her property, left intestate, would devolve upon her legal heirs. However, thee is a special provision if the property is situated in Kerala. Please see Rule 15 of the Hindu Succession Act.
Section 15. General rules of succession in the case of female Hindus.
(1) The property of a female Hindu dying intestate shall devolve according to the rules set out in section 16,
(a) firstly, upon the sons and daughters (including the children of any pre-deceased son or daughter) and the husband;
(b) secondly, upon the heirs of the husband;
(c) thirdly, upon the mother and father;
(d) fourthly, upon the heirs of the father; and
(e) lastly, upon the heirs of the mother.
(2) Notwithstanding anything contained in sub-section (1),
(a) any property inherited by a female Hindu from her father or mother shall devolve, in the absence of any son or daughter of the deceased (including the children of any pre-deceased son or daughter) not upon the other heirs referred in sub-section (1) in the order specified therein, but upon the heirs of the father; and
(b) any property inherited by a female Hindu from her husband or from her father-in-law shall devolve, in the absence of any son or daughter of the deceased (including the children of any pre-deceased son or daughter) not upon the other heirs referred to in sub-section (1) in the order specified therein, but upon the heirs of the husband.
STATE AMENDMENT
Kerala.--
Amendment of section 15.--In the Hindu Succession Act, 1956 (Central Act 30 of 1956), in section 15, after clause (b) of sub-section (2), the following clause shall be inserted, namely:--
"(c)" any property inherited by a female Hindu from her pre-deceased son shall devolve, not upon the other heirs referred to in sub-section (1) in the order specified therein, but upon the heirs of the predeceased son from whom she inherited the property."
[vide Kerala Act 17 of 2016, sec. 2].