I would get some sevins dust and treat the coop with it. You can get it at your local hardware store. I would also dust the birds liberally with it as well. Ensure they do not breath it in. The problem with most lice is they come out at dark and infest the birds at night then retreat at dawn to hid in the coop. They will literally suck the life out of the birds at night. I would do this as quickly as possible as all your other birds are at risk of a similar fate. Unfortunately, You only find out you have an issue after a problem arises. Another thing you can use is Diatomaceous earth used in the same way. It is non toxic and will not harm your birds at all and is even recommended to sprinkle on their food to rid them from internal parasites. This would be my first chice of treatment.
http://www.theprairiehomestead.com/2014/01/diatomaceous-earth-homestead.html
