Is testifying that the grandmother is not among the heirs, and the father's waiver of her inheritance on her behalf without a written document, considered false testimony, and what is its expiation?

Your testimony before the judge, limiting the heirs without mentioning your grandmother, is a false testimony (shahadat az-zoor), one of the gravest sins. Its ugliness may be compounded if accompanied by a false oath (yameen ghamoos). You are required to repent to Allah Almighty and to restore the right that your grandmother lost, which is her share of the inheritance.