The grandfather’s wife is a mahram

Praise be to Allah.

The grandfather’s wife is a mahram, so it is permissible for you to see her, shake hands with her, be alone with her, and travel with her.

Shaykh Ibn Baaz (may Allah have mercy on him) said:

The grandfather’s wife is a mahram, whether the grandfather is on the mother’s side or on the father’s side, because Allah, may He be glorified, says (interpretation of the meaning):

{And do not marry those [women] whom your fathers married} [an-Nisa’ 4:22].

This includes fathers and grandfathers on both sides, on the mother’s side and on the father’s side. Therefore she is a mahram to him [the grandson], and he may travel with her and be alone with her; there is nothing wrong with that.

End quote from the website of Shaykh Ibn Baaz.

And Allah knows best.