OK, Ignore the nitrozorb, it's not necessary, but I'd probably try to avoid messing with your filter if at all possible and I doubt it's hurting, so I'd leave it alone unless you're messing with the filter for a reason.
Balajake is quite correct to ask your nitRITE levels. NitRATE, as said above, is not all that toxic unless the levels get very high. I have more nitrate than that in my tap water and fish are perfectly happy in my tap water tanks.
Nitrite however is quite toxic, and the bacteria that break them down can be a pain at times about growing back.
I suspect that you've lost a lot of the live bacteria and that's why you're having problems. Essentially puts you in a fish-in cycle. Not the end of the world, just harder work at the beginning. If your friend has an active filter, that's been on a tank stocked with fish, then it should certainly help, as that would take a lot of the load and pressure off.
Trying to clarify how this happened, how long was the tank empty of fish/media dried out before you took over, and how long was the tank empty afterwards? Also, what did you use to clean things? Generally the advice is to only clean filter media in water from the tank it's in. The tank itself you should be fine cleaning, so long as the cleaner wouldn't then get into the filter and kill the bacteria.
Your synodontis catfish would show signs like reddening of the gills, gasping at the surface, lying on the bottom (not really the best clinical sign to use to diagnose a problem in a catfish) and general lack of energy. It may be suffering as it's the biggest fish in the tank and therefore needs more oxygen than the other, so a gill problem would be a challenge to it. It may simply be more susceptible to the water problems that you're having at the moment.
If you read the advice on fish-in cycling in the beginners section you'll find a bit about water changes and keeping toxic levels down, ideally as low as possible. Remember that if you do a 50% water change you can only really expect a 50% drop in the toxin levels as well, so sometimes much larger water changes are needed if the tank is having trouble. I've done 90% changes before a few days running after a tank had trouble.
