I feel that people have gone over the top & 'jumped down your throat' unfairly, without considering different situations, knowledge, etc. (Someone even said something about irresponsability for not getting a vet out for Coggins - is that even an issue in Australia?? I & everyone I know must be irresponsible too on that note)
However, it doesn't help attitudes towards you when you change your story, esp after getting stuck into someone for not reading it right, and given your story, I do agree with the basic sentiment, if not the judging & 'flaming'.
The story as I see it; The horse had a serious nail injury & infection, which was not treated appropriately for (?) months. You now know this is an emergency, not something to 'wait & see' about. It is no argument to say you've heard of others that have pulled through without veterinary care - not every... heroine addict dies of heroin either, but it doesn't make it a safe choice.
Now it may be 'too little too late' regardless of how much you were willing to spend on the horse, but as you're not willing(or able - not judging). It's also not a good enough argument to say the horse was a freebie so not 'worth' the money - the horse doesn't know or care how much she cost you - she's still a horse & relies on your family for her wellbeing, to whome the responsibility falls. For the sake of the animal, who has already been suffering for months, I don't think a half hearted attempt of antibiotics & stable rest is good enough - & will likely just be a waste of your money & prolong her suffering.
So the way I see it, you have a few choices; keep on doing what you're doing & most likely get(& give her) what you've been getting, or worse. Get the horse into an equine hospital for adequate care & cross your fingers. Find her a home where the owner is willing to do that for her. Or put her out of her misery.
Where abouts is the infection & what has the vet said anyway? I presume you're here for a second opinion? Maybe because the vet has said something similar & you were hoping they were exaggerating? I suspect on the contrary you got a stronger response than the vet gave, because for one, I'm not sure if they're even allowed to be so blunt on the matter of putting animals down.