I'm so sorry for your loss, it's terrible. It really rips your heart out.
When my then 13 year old cat got sick, she started to withdraw. She was hiding under the bed, didn't eat anymore and she was also vomiting a lot. And her breathing was laboured too, it was like she was air hungry.
It was on a weekend and we drove her to an open vet clinic. There they told us she was jaundice, had yellow gums and eyes. They x-rayed her and it turned out she had a large mass on her liver, most likely a malignant tumor. She was so sick and miserable we decided then and there to put her out of her misery. We definetly didn't want to put her through uncertain cancer treatments. She was put to sleep for good in my lap and we cried like babies.
I "saw" her in the corner of my eyes for a long time after at home. The emptiness was so huge and I felt like I had amputated a big part of me. I still think of her and the joy she brought us all even though this was over ten years ago.
I'm sorry for you your friend passed in such way and I hope you have loved ones to get support from now. And I also hope one day the sadness will lift and you can cherish all the wonderful memories your cat gave you without feeling the weight of a heavy grief.
Big hug to you.