He refers to her as Baa-san, which translates to "mother" or "grandmother". Grandmother/Grandma would be more suitable here since they are already old. He refers her like this as their names were not revealed in the whole manga and he cannot call her "manager" even after marriage.
Honestly this is a perfect ending considering. She did what she set out to do, she saved him a bunch of times & she finally died peacefully. I think this is the way to go
Dude why are you saying this isn't a happy ending? They clearly lived together for a long, long time, probably married with kids since he's calling her "grandma". 10/10 ending as far as I'm concerned.