Drop the water tamp temp 75. Treat with salt dosed at one tablespoon per 5 gallons of water. This will slow it down, but not cure it. Treat with kanamycin and furan 2 asap.
Ducklips (Flexibactor columnaris) is a gram negative bacterial infection, so an antibiotic medication like Maracyn, or Cephalexin should be used.
Ducklips is most virulent at temps above 82'F so one of the first things to do is to gradually bring water temp down below 82'F.
Once the mouth has become severely distended its really all over for the fish, and I find its best to get it out of the tank and in hope that the other fish don't become infected.
I thoroughly vacuum the tank, then dose the it with Hydrogen peroxide to knock down the bacterial population. http://www.thekoikeepers.com/koihealth/dreadedflexi.html