I've been reading on columnaris as i had a very serious virus wipe out over 40 of my guppies and their strains died and now I've been beating with with simply keeping the water clean and keeping they're temp no higher than 75F. I read this bacteria multiplies much faster in warmer temps so don't use the "ich high temperature" treatment for this or you'll have the opposite effect. So far NO more guppies of mine have it.
As for the tiny fry, get a breeder trap(only about $5) to keep the fry in your bigger tank so you can medicate your sick fish more. Even still, wouldn't you prefer to save this adult over the fry?