All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The husband of the grandmother is not a Mahram for your wife, rather he is a marriageable man to her, and the ruling that applies to him with regard to looking at him or being in seclusion with him is the ruling of marriageable men.
Allaah Knows best.