It's real important, for the fish in question, and especially for the fish which have not shown the symptom yet, that you make sure your water quality is superb. Lots of aeration, minimal organic material, zero ammonia and a stable pH.

Let us assume that a parasite caused the initial lesions. Treat agressively and be sure that whichever medications you choose that your spectrum includes a seamless protocol to intercept Flukes. Flukes will survive single treatments of Formalin, in fact, they'll survive single treatments of ANYTHING since the embryo remains safe inside the abdomen of it's dead adult. One of the keys to Fluke eradication is the second treatment to intercept emerging larvae.

You might swab the wound once, but don't go overboard.

The other main cause, is of course, the koi in question has physically damaged itself against an object or the sides of the pond ... Koi if suddenly startled do tend to scatter in every direction which of course includes running into the sides of the pond with such force that it actually smashes the jaw bones which if left undetected causes secondary infection to set in which then results in the mouth of the fish becoming extremely bad and infected. Try to avoid this by checking your koi regularly for such damage .. This is quite easy to spot when you are feeding them and their mouths are clearly visible.