Nitrate titers at 40 ppm are at the threshold for long-term damage, but not immediately injurious. Water changes should reduce that level and in the future keep the nitrate level below 20 ppm if at all possible, at about 10 ppm seems ideal.
Salt is a waste for NTD, it will have no effect. If it is NTD, sacrifice the fish immediately, or at least get it out to another tank. If it dies in the tank, the other fish are at risk even if they have not yet cotracted the disease.
Salt in a tank of neons is poor aquairium practice - you are increasing the TDS and osmolarity to no benefit on a fish from very low TDS water. It will not kill them outright, but it won't do them any good either. Salt as a tonic is one of the most pernicious myths in the hobby. Salt without a diagnosis of the condition is a shot in the dark, just as inappropriate as any other med without a diagnosis.