Can we take a look at your stats in ammonia,nitrite,nitrate and ph, corys should be kept in groups of no less than 3, and tetras should not be kept in less than a group of six, but if you did add more tetra's your tank would be overstocked, so i would think of rehoming the tetra.
The mouth could she of banged it on something in the tank, does it look fluffy in appearance.
If she hasn't knocked it on something, is sounds bacterial, and you will need a bacterial med, i would perform a water change before you add the med.
The lfs is telling you porkies about the fish.