I am so sorry. Our family cat, Sammy, had lung cancer. (Before you ask, no one around him smoked, nor did Sammy smoke har har.) We put him to sleep six years ago, and I still miss him. We made that decision based on our vet's advice and the advice of the two specialists our vet recommended. I can't give you advice on what to do, as your cat's situation could be different than Sammy's. Chemo might be a viable option. Ask your vet if he knows any cancer specialists. Get second and third opinions. I drove two and a half hours for a third opinion. Everyone I spoke with said that chemo was not effective on his type of cancer, and that I should watch his eating habits. Sammy was with us about three months after his diagnosis, and then he stopped eating. At that point, we brought Sammy in, and our vet recommended euthanasia. We took his advice, and I think it was the right thing to do in our situation. This is very sad, but I think it's important to know -- we didn't know. Euthanasia is instant, which is good for your cat. This is just something you should know so you have some time to say goodbye to your friend. We had some time alone with Sam in the clinic to say goodbye, but it didn't feel like enough. Maybe it's never enough, and I'm crying now, so I can't imagine this is making you feel very good.
During the ordeal, I learned that cancer is not uncommon in cats around the age of 13 or 14. It's a sad fact, and it's not easy for you or your friend. After Sammy was diagnosed, his behavior changed significantly. What was most remarkable was the way he would spend every minute sitting next to us. He used to be a very independent cat, but that changed when he got sick. I don't know if he was telling us he was ready to go, or was scared and needed company, but I am certain he knew what was coming and wanted to spend all of his time with us. Maybe I'm anthropomorphizing a little too much.
Call me if you want to talk about it. I'll listen.