I cannot understand how anyone could do this to a pet Mccfluff and how you might feel. I can understand you wanting to cry.
When I was at my lowest (before I came to the bar) and I was kicked out by the man I was living with, I spent 2 nights sleeping in my car (not pleasant I can tell you) before I found a safe place for my cat. She was the thing that kept me going when things got really tough. If I hadn't had that responsibility god knows what might have happened. Happily, after 2 weeks, I found somewhere and we were reunited. We are still together now and that was several years ago. But there was nothing I would not have done to ensure her safety and comfort.
I can never understand people that take on a pet and then fail to afford it the comfort and care it deserves. I don't think there is anything you could do, this is down to the underclass who have no comprehension of responsibility.
This is sad beyond words, but I regret that you have done all you can in buying dog food. Perhaps if the guy who "bought" the dog comes in again, you could have a list of good rescue centres available for when he inevitably tires of the pooch.