Makes me wonder what the vet actually researched.
Looks like guessing to me.
Not eating because of a resting period due to a cold period still doesn't tell you anything about the sore.
Like i told you before, it could be bacterial or fungal related.
Without proper research (by swabbing) you can't tell me you know it isn't the cause.
Fact that it doesn't heal up and you're frog is getting worse tells me that you need to let a proper vet look into it,
so you know where you are dealing with.
With the right medication it doesn't have to be that big of a deal......