Freshwater feeders are very fatty and will cause health problems for SW fish in the long-run.
Living in the ocean can a porcupine puffer eat a freshwater fish? No. They don't go together.
When I had a porky in my last tank, I used to go to the grocery store and buy what is called chowder mix. It would have a mixture of things like cod, squid, shrimp, etc. This kind of thing makes an excellent meal for larger fish. You can also refreeze what you don't use right away.
Also, at your LFS, ask them if they have any "bigger" foods. Most foods will be available in larger sizes. Things like krill and mysis shrimp are two that come to mind.
Although you puffer might look like he is enjoying that feeder fish when he eats it, it will come back to haunt him(and you) if it continues.
Hope this helps!