Is there any grass for them to eat at all?
If not, they to get to the condition you describe basically are starving, sad.
OK... you have hearts of gold and them some. To start these animals off though, due to no food or very little only give hay, NO grain. Even small amounts of hay will be an adjustment that their digestive system needs to make. Horses are made to graze, or eat approx. 20 hours a day while meandering... so even to much hay can cause digestive upset at first. DO not feed them a high protein hay, even though tempted, there bodies can not handle a very rich food right now... a nice grass mix or combination hay they will thrive on as they have had nothing in a long time. Animals do not get this way in a short time... it takes months of neglect for an animal to resemble walking carcasses.
As many horses as they have they need that many piles of hay and then a couple more so squabbling still allows the weakest/lowest on the totem pole to get some food.
A flake for each pile a few times a day while you are there would be best to start them off. Once they start eating and get some digestive juices flowing... increase in small amounts... the idea is to have hay in front of them as much as possible. Eventually, this is over a couple of weeks you would like to be able to see them fed 3 -4 flakes(pieces) per horse over at least 3 feedings, preferably 4 feedings. This amount of hay should actually show great improvement on their thin bodies. I would hesitate to add grain at all as if no way to regulate who gets how much... you could get a grain overload on the bully/dominate horse and have a colic these people can't afford, nor can you to be held responsible.
I would also make sure they have available salt blocks, a mineral & a plain white lick. Fresh water is a must and plentiful supply.
Unless you are going to do the actual feedings and supervise I would not give more than the hay and salt at this point. Carefully think about the worming in a few weeks once the horses have had a chance to get some food in their guts and their guts working again.
I would also not "set-up" any account at a feed store for them to use, but you be the one to call or pre-setup a delivery of needed products. This protects you from outrageous bills and also ensures the horses get the feed, it is not sold off to others.
I had a "starvation" case myself. These were the guidelines the vet gave me to follow. For my 1 horse, averaged 10 bales of hay a month
or figure 2.5 bales per horse per week
. This was based on good approx. 40 -50 lb solid bales.... it may be cheaper to purchase by the ton for hay only if there is a dry place to keep it (at your house) and then distributed accordingly.
Take some pictures to document what you are seeing and the improvement they make. You should see a difference in about 2 weeks or so of starting to feed them. Remember the inside must first be nourished before the outside shows anything at all.
Bless you for caring and making the effort to give these innocent animals a chance. It will at some time be returned to you ten-fold by someone else doing a act of kindness. ETA: agree with Slim... If you do anything more like worm or feed grain, etc... get it in writing from the owners that you are not held responsible for adverse effects to the horses. Today you must protect yourself from lawsuits... sad when you only want to do good for the animals.