I'm sure it's different in different areas, but from what I've been told by vets & the likes, and from what I've experienced, it seems that *generally* only unhealthy individuals &/or with poor nutrition are really susceptible to lice.... & donkeys! My donk used to get lice from birds every year at the beginning of spring, so that if he was left untreated, he'd rub his whole coat off on his sides & shoulders! This is apparently common in donks. Not that I can say for certain he was in good shape & nutrition either, as this was years ago when I wasn't very knowlegable about all that & he did have a heart prob.
I've also kept my horses in paddocks with licey horses & never had a prob with mine. I wouldn't personally worry about this horse being with the others, so long as they were well looked after & in good shape. I certainly wouldn't like to keep the horse locked up permanently, as this is not good for him at all.
I use a powder called 'Pestene'(sp?) which you just sprinkle along the effected animal's topline - or over chickens or other animals who are affected by lice. You can actually sprinkle this around bedding too, and if you suspect your own horses may be susceptible to lice, you can always give them a treatment too. As previously mentioned, it's a good idea to do a couple of follow up treatments a week or so apart, to catch any beasties which may not have been in the active phase to get got the first time.