I would defintiely advise your friend to feed her boy garlic. I use the gralic granules, you don't say how big her horse/pony is but the recommeneded dose is usually on the tub.
I'd also recommend using essential oils. Lavendar, Bergamot, German chamomile, Roman chamomile, Tea Tree, Geranium, Hyssop, Citonella, Lime and Garlic are all helpful.
Be careful with the garlic, it is pretty strong on the skin so also use chamomile or yarrow. With oils I probably wouldn't mix more than 3-4 max and mix them in a base oil or gel (like Aloe vera)
I've used the below essential oils mixed in 100mls of base oil (I used coconut)
Apply it morning and night, and if may be a few days before you see an improvement - stick with it!
Your friend could also consider getting a Boett rug - stops the midges being able to bite him in the first place.