Is a person considered a Mahram (unmarriageable relative) to the maternal grandmother of his wife?
If the matter is exactly as what is mentioned in the question, the grandmother is considered a Mahram to the husband of her granddaughter. Proof for the foregoing is Allah's Saying:
Forbidden to you (for marriage) are: your mothers
to His Saying (Exalted be He):
...your wives’ mothers
The ruling which applies to mothers also applies to grandmothers and great grandmothers. Consequently, it is permissible for a maternal grandmother to take off her Hijab (veil) in the presence of the husband of her granddaughter. (Part No. 17; Page No. 374) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.