I am sorry for your friend I kind of know how she feels. I lost my beloved cat back in Dec...I am still having a lot of guilt and just feel like hell on it..I don't know how you can help other than what you suggested to her and still doing all you have..being there.....You may want to drop over more so she can't isolate....she may ...Been there...Also DON'T tell her she will or can get another cat someday....I find it annoys me and diminshes how I felt about my cat. Also, when she does bring it up REMIND her of all she did like taking kitty to the vets and just being a good cat mom (in all the ways you know she was) it will help...Mostly be there and keep reminding her..when she brings it up....she did more for her cat than a lot of people do....many vet bills and so on..I am very sorry this is sad