Hate to say, but, you really should have researched the puffer more. Starfish are one of a pufferfishes bestest (slang, I really do know how to speak) delicacies. Puffers eat some types of coral, starfish, crabs, snails, hermits, and most inverts. These are it's natural food. It has a mouth like a bird beak, and 10 times more powerful. It will crush a shell in a split second.
You said you have seen the porkpuffer peck at him, and he is getting soft an falling apart. That is what stars do when they are stressed. Imagine being eaten alive, slowly. Echinoderms have the ability to break up into many parts when stressed. But, too much is too much.