Sorry man, that's so shitty.
I had a pit bull that my parents were feeding dog food from walmart, he actually lost the ability to use his hind legs from the shitty food. My grandmother was so devastated that she just decided she was going to make ALL the dogs homemade foods, shed spend hours one day a week making chicken and rice and certain vegetables into a patte like substance. The pit bull actually gained back some mobility in his hind legs, which was encouraging, but he never was able to run around again, but he was happy with the little hind legs wheel chair thing my folks got him.
Food is super important, it was a hard lesson we learned. For my animals, I go to PetSmart and buy Blue Buffalo brand for both cats and dogs and we've not had any issues.
Sorry for your loss, it's never easy.