Many Kudos to you for your willingness to help this horse
I see you're in New Zealand, so the products may not be quite the same but:
1. First cut off all the fetlock hair and shave the hair down to a crew cut in the back of the fetlock joint, if the horse can stand to be touched with the clippers. Regardless of bacterial or fungal, the skin in that area needs air to dry up Mud Fever.
2. If her legs are covered in mud and/or manure, wash them either with some sort of microbial wash like Betadyne or an antibacterial dish soap. I use Dawn dish soap because that is what the rescue folks use to wash ducks and other fowl whenever a big oil spill happens
Pat as dry as possible with paper towels, so you can throw them away.
3. The cheapest & most effective topicals that SHOULD work (nothing's 100%:( is an equal mix of Hemerhoid ointment/Triple antibiotic ointment/diaper rash cream.
Clean legs and apply goop twice a day. I will pull the scabs off if they appear ready to come off and as long as there's no bleeding; that means the sore isn't healed enough for the scab to come off.
It's important to keep the legs clean and healing as Cellulitis can set in. Keep an eye out that the legs don't start to get puffy.
Back to the founder. It's grass founder so the odds are really good the horse has insulin issues; insulin issues can oftentimes compromise the immune system<-----weak immune systems can lead to chronic cases of mud fever, rainrot, thrush. See where all this is going?
That means no grains whatsoever. The horse needs to be put on a low starch diet, quality grass hay and either no pasture (put her in a dry lot if possible), OR put a grazing muzzle on her and limit the time on pasture.
What season are you in right now? The U.S. Is heading into Fall and this time of year is actually as bad or worse for grass founder.
We get hard frost at night and fast warm ups the next day. That equals a surge of fructans thru the grasses which, in turn, is what causes the horse to founder.
I know this isn't your horse and your responsibility only goes as far as you have money and time. Hopefully you can re-arrange this horse's eating schedule without dipping into your finances or causing you too much extra time