Hello,
Thought I may be able to help you here.
First of all, make absolutely sure it's lice. They commonly hang out under the mane & forelock, as well as all over the body in more severe cases.
Isolating the affected horse as you did, is the best thing to do. The lice will live in the horse's environment so disposing of the entire stable of bedding when you've finished treating would be a good idea. Wash everything (now and regularly throughout treatment) that belongs to the horse i.e. rugs, numnahs, grooming kit, headcollars, other tack. An infestation will take a good few weeks of treatment as you have to make sure all of the adult creepy crawlies have been killed off, so they don't lay eggs. Also need to treat the baby creepy crawlies that have just hatched so they don't lay eggs too. This breaks the cycle. Treat the horse at intervals and take the manufacturer's advice (if in doubt contact them).
Treatment wise, you can buy several products at your local tack shop. These products are usually permethrin-based. Look for Dermoline, Deosect or Switch. Additionally, lice infestations are far easier to treat of you clip the horse but beware, some people don't like clipping their horses. An advantage is that very soon the winter coats will be shed so this will help to get rid pf the problem. Groom the horse thoroughly a couple of times a day (if you can) this will help loads.
I wouldn't use the mites injection for treating lice. Although is it commonly used in horses, it only tends to be used for treating mites. It is not licensed for use in horses and because of this under the rules of the prescribing cascade, you have to use a product that is licensed 1. for use in horses AND 2. for treating lice. If there is no such product then using the mites injection would be ok - but you would have to sign a disclaimer type of form to authorise the vet to use it. The owner may not like you doing this on her behalf either.
It would be useful to investigate why the horse has developed this. Is it a sick horse? old? underlying disease? not groomed very often?
If you get stuck or it doesn't seem to be working, get the vet to have a look at the horse.
Hope this helps and keep me posted x