I'll try to be constructive, but it's pushing it:
The dog should have gone to the vet the day of the incident, no "ifs" "ands" or "buts" about it. Since that wasn't done, the dog obviously needs to see a vet now and that's not open for debate. If money is an issue (which it apparently is), like others have already said, find someone to work with you. You said you found someone to do the initial exam for $50, get it done. That visit will determine the next course of action whether that is treatment (which probably won't happen since nobody involved in this seems to have any money) or just euthanasia since the dog cannot be allowed to suffer. Obviously some vets are going to be more flexible than others.
I'm an animal cruelty investigator - depending on where you live and the laws in the area, you or anyone involved in this could be charged with animal cruelty for neglecting to care for the animal. I understand you are financially challenged, so am I. But the hard fact of the matter is that keeping animals is expensive and is a responsibility. How do I know? I've currently got six furry critters living with me in my house

I deal with cases all of the time like yours where someone is trying to do the right thing, but the owner is refusing. We just did a case last week where we are bringing the person up on charges because their dog got torn apart by another dog, and she was just leaving it out on her porch. Why? No money, go figure. She's going to pay the price now (even though it's a misdemeanor out here - sigh.....).
If the owner isn't cooperating with you, start by calling local animal control and advise them of the situation. Call the local police/sheriff's if that doesn't work. They might not be willing/able to do anything, but a lot of times if they refer a case to animal control, they're more willing to do their jobs (I was in law enforcement for 16 years - ask me how I know). Next step would be to contact your local animal rights/welfare groups in the area like the one I'm looking for (you may actually want to call them first). Many of them offer free or reduced cost services for people in your situations or they may be able to refer you to someone who can. Surrendering the animal to a welfare organization may also be an option since some of them will take care of the medical treatment of the animal before placing it for adoption (not all - depends on their budget, etc.). My organization is pretty good that way - we've got full time vets, surgeons, techs, etc. on staff and we bend over backwards to rehabilitate the animals. The dog I mentioned earlier was seized by us and the sheriff's and got treatment. The dog is good to go now. Even if they wont' care for it, they will take care of euthanizing the animal if it comes to that. Some charge a surrender fee, some don't.
Good luck with it either way.