I am so sorry
The other horses ran her all night because she is in a weakened state. Even thought they are still operating on the that 2,000 year old "survival of the fittest", it still makes ya want to put a bullet in all of them.
Even though she is now very thin, she is a prime candidate for metabolic issues. Her immune system is not only out of sync from having the foal but it is in weakened condition.
There are ways to get her weight back on without feeding her grain or grain products.
I'd say she has lost weight because of feeding the foal and the three running her wouldn't let her eat off the roundbale, if you have one out. Even if you're throwing hay, I'd bet a lot of $$$$ they weren't letting her eat.
That means everything she has in her was going to the foal and she is literally starving to death, thanks to the three running her.
I would also bet that, by now, she has ulcers of some sort; those will also contribute to weight loss.
If he has a barn, is there any chance the farrier that used to own her would take her back, at least until the abscesses have had time to heal and she can start putting weight back on?
You're in a tough position without a barn, to where you can put her in out of the weather.
I don't know how remote you are, therefore how good your vet really is when it comes to being up-to-speed on special issues like what you're dealing with. She needs to be on a diet that gives her plenty of healthy fat to gain weight, yet nothing that will aggravate the ulcers, I suspect she may have (grain is bad for them).
I wish you well in a speedy recovery for her