I feel sad for your mother too, as it's a real big thing to lose a loved pet. Even though the cat is old and you know that it's time is almost done.
You never choose your cat but the cat chooses you. I do hope your mother can be happy for the time she had with the cat who chose her.
Yeah, she is very sad right now, but she also knew it couldn't last a lot longer. She's happy for the way she passed away and for the life they had together :-)
It's good :) Everybody is allowed to be sad after years of happiness with a cat.