No body is going to tell him about the likelihood of a serious bacterial infection??? When this happens suddenly and drastically, it is a sure sign of an internal infection. The jardini could be dead within 24 hrs of such a sudden onset. This is why it is so important to keep a supply of meds on hand.
Treat the tank as soon as you can get some quality meds. Do not rely on cheapies and do not use on Pimafix or Melafix. If you can find Lifeguard All-In-One, that will be your best bet.
Do not add salt over what is normally used and what ever you do, DO NOT INCREASE THE TEMPERATURE. The bacteria multiply faster at high temperatures. The fish has a better chance of fighting the infection at a lower temp with a smaller bacteria population rather than at a high temp with a rapidly multiplying bacteria population.