Sorry, but your friend's word is a load of bunk. I have not anything nicer to say about this. Salt will not prevent anything from causing issues towards your fish. In fact, adding salt on a permanent basis will harm your fish more than help. Why? Salt adds electrolytes. The osmoregulation of the fish can handle only so much and any added electrolytes will harm the fish i.e. those who live in soft acidic waters where the conductivity is very low comparable to hard alkaline waters.
The only benefits you can get from salt is destroying the protozoans once they are introduced in your tank and neutralizing the nitrites which is very damaging to the fish. But I will not suggest adding salt in your tank unless necessary. If you're using iodized table salt, aside from the purposes mentioned earlier, iodized table salt prevents goiter which is from lack of iodine in the water. Yes, fish needs iodine as much as we do. There is very little trace of iodine in the water. Anything about iodine and additives being harmful is also a load of bunk. These myths have been continuously parroted and encouraged by companies to ensure they gain profit from the so-called 'aquarium' salt. Table salt in itself is cost effective and as effective as the 'aquarium' salt.
Yes, I know I just entered into another salt debate but I just don't bear it when someone keeps parroting myths without proving the iodine, etc can in fact harm the fish. One way for people to direct the blame on salt is failing to admit they did not dissolve the salt before adding it to the tank. Salt has to be added slowly and carefully. Direct contact of salt grains can burn the skin of the fish. The sudden surgence of the electrolytes can cause osmotic shock to the fish thus killing it in the process.
Tcarswell, your last post indicated you are adding the salt the wrong way. Always dissolve and add slowly but you are also adding it for the wrong reasons per the word of your friend which are all utter nonsense.