Some people are successful using a strong antibiotic able to kill Gram-negative bacteria.
I was "never successful" treating it, so rather than take a chance of it spreading to other fish, I tended to euthanize.
Being it is your only fish, it is reasonable to try to treat, if the disease is very advanced, it usually kills within a few days, so you won't need long to wait to know, if its working or not.
If you do treat, and it dies, "I" wouId add a gallon of bleach to the tank letting it sterilize filters, media, everything the fish had come into contact with, before adding any new fish.
When duck lips begins it is sometimes not obvious, the fish hovers in one area, doesn't eat, often color up brightly, but the stress of capture and moving to a new tank can bring on the atrophy in the jaw very quickly as it spreads.
Your LFS may not have noticed the symptoms, they may have have been very subtle.
Note the only slight projection of the jaw in the shot below.