Just reiterate the fact that it wasn't her fault and that he's in a better place right now with other cats, and was happy with her in his time here on Earth. Let her know that he will be watching over her and will continue to live on in her heart and through cats that she will own in the future. Remember the good times and that 11 years is a happy long life for a cat.

I have pictures of my 2 childhood cats that have passed on on my refrigerator still and I think of them often. Perhaps a picture or a nice memorial stone in the backyard would help. Good luck! She's lucky to have a friend like you.