It's TOUGH to lose a family pet, I'm nearly in your friends shoes right now. We have had this cat (I caught wild as a kitten) and he has been a remarkably smart and loyal cat. Always well-behaved; our kids grew up with him. Right now he has an out-of-control tumor growing outward from his side, it's is awful looking thing, but I'm taking care of him everyday and assessing his quality of life.
He is not in pain, he eats and drinks as he likes, uses his litterbox. He seems very happy to be at home and with us. As long as that continues he'll be fine here. As his health diminishes, and I know it's coming, I'll have him euthanized as comfortably as I can. He's given us many years of companionship and pleasure, I'm sad to lose him, but happy to make his departure easy for him.