Hey,
I have read your query and would like to answer your questions but before that lets examine the validity of the will. As mentioned by you that your grandfather got his share of property through partition which means he is the sole owner of the property after partition, therefore, it will be referred as his separate property or self-acquired property, therefore, he has a right to transfer the interest to any person as per his wishes.
Some grounds for which a will can be challenged are as follows:
· Section 61 of Indian Succession Act, 1925 states that any will which has been obtained through fraud or coercion can be challenged.
· Section 59 of Indian Succession Act, 1925 states that any will made by a person of unsound mind, minors, ill or intoxicated can be challenged.
· Section 89 of Indian Succession Act, 1925 states that a will without a certain object or subject would be invalid. For example any irrational terms with no clear beneficiary, ambiguity or confusion with respect to the intention declared by the testator in the will.
· Section 124 of Indian Succession Act, 1925 states that a contingent legacy can take affect only on the happening of that contingency. A contingent will whose execution is dependent upon the happening of a specific event or condition.
· Section 127 of the Indian Succession Act, 1925 provides that any condition of the will which are illegal or immoral can be declared void, or if the conditions are against any public policy then it would be declared invalid.
If there is no infirmity in the will as per the provisions mentioned above then the will can not be challenged and your father can reach out to your grandfather to deal with this situation amicably.
Regards
Parth Chawla