A woman said to a man, "Your mother's husband's father is my husband". How is the woman related to the man?

Let's break down the statement from the perspective of the man:
"Your mother's husband" → The man's Father.

"Your mother's husband's father" → The man's father's father, which is the man's Paternal Grandfather.

"...is my husband" → The woman is married to this grandfather.

So, the woman is the wife of the man's grandfather. This means the woman is the man's Grandmother.