You're in New Zealand, so I'm not sure what all is available to you.
1. Here's the UK link that talks about keeping the horse inside at dusk and at dawn but she's in a boarding situation so that's probably not possible.
I honestly don't know if their Boett Blanket is successful. I know I first learned about sweet itch seven years ago from this website. They must be selling the blanket as the website has recently been updated. The National Sweet Itch Centre and the Boett Blanket
2. Unless the horse has wide-open seeping sores, I would bath her in a medicated shampoo then rinse her in a mix of apple cider vinegar & water. 100% pure apple cider vinegar
Zinc oxide is a huge help on open sores. In the U.S.A. zinc oxide can be found by itself or in diaper rash cream in any general store like WalMart or drugstores like WalGreens, CVA's, etc.
I would make a 50-50 mix of zinc oxide with whatever human antibiotic ointments you have available and dab the mix on the worst open sores - paying strict attention to the belly line.
It's a yukky mess and needs cleaned off the horse, then re-applied, every day
If the horse's entire body is pretty much an open sore, I sure hope the vet gave your friend some sort of medication that can easily be applied to the entire body.
Here, in the U.S.A., double-dosing with pure Ivermectin has been successful in eliminating sweet itch IF the horse is showing signs of Onchocera worms, a/k/a neck threadworms.
They aren't really worms but microfiliae that burrow in the sking after the horse is bitten by Midge Flies. Equine-Onchocerciasis in Horses
I'm probably not much help as, from what I've read, you folks across The Big Pond, have a horrendous time dealing with midge flies. Much much worse than we have