I mean it's not like he's her Father or something.
Some people call their mother and father-in-laws "mom," "dad," "ma," "pa," "pop," etc. Grandpa Lou probably told Didi he wants her to think of him like a father. After all, they live in the same house.
Exactly, it's pretty common.
My Grandma used to call my Grandpa's mother "Mom."
I noticed that she actually calls him "Grandpa" instead of "Pop" in some of the first season episodes.
It's not uncommon. My dad calls my maternal grandparents "Ma and Pa"
He's her father-in-law.
He's her father in law. Just like how Angelica called her aunt Didi even though Stu is her biological uncle and didi is married to him.
Is that weird? I always called my Uncle's wife Aunt Linda