Please can you advise on the idat period for a woman who has become widow at age 76.

Ruling 2528. There is no ╩┐iddah for a girl who has not completed nine lunar years nor for a postmenopausal woman. As you mentioned above that she is 76.
Otherwise iddah for divorce is 3 months and iddah for death of husband is 4 months 10 days.