Puppy kicked by horse, slipping into coma?

If he is slipping into unconsciousness he most likely has a severe head wound resulting in brain swelling. I have seen many cases of brain trauma treated with IV fluids, antiinflammatories, and Mannitol make full recoveries. If treated promptly and aggressively they an have an excellent chance of survival with no lasting consequences, but if left untreated the brain swells until it has no more room. This results in severe hemorrhaging and seizures, then death....there is nothing your friend can do at home, they do not have the medication or knowledge to handle an injury of this magnitude. Please urge them to get to a vet or emergency vet clinic, they can apply for Care Credit at a vet office and if approved can pay off the card at the rate that works for them. But to not even make an attempt, yes, they could very well be found guilty of neglect.

Yes, the person could very well be charged with animal cruelty; by not taking the dog to the vet, they are potentially leaving him to suffer - he may be in pain and it's cruel & neglectful to leave a dog in this state.

Yes, call animal welfare and report them.