You say you want to get water parameters in check -- what are they?
Just about any salt tank should be:
Temp: 80-83 degrees F
Regarding water changes:
1. Check the salt level in the tank first. If he's just been topping off, the salt level could be way out of whack (for example, salinity = 1.032). If that is the case, you don't want to do a massive change with water that is salinity = 1.023. That's not good for the fish. Instead, you'll have to mix in smaller batches of higher salinity water (1.030, 1.028, etc.) and work your way down to 1.023.
2. Don't use tap water! Tap water usually has at least traces of phosphates, sometimes a lot. This drives algae in a salt tank. He needs to use RO/DI water. If he doesn't have a water maker then he'll need to buy water. You can have water delivered to the house from Deer Park or one of those -- that water has usually been through the RO process, with UV sterilization, and distillation, and chemical treatment, etc.... Tropical marine waters are known for being devoid of nutrients.
3. Make sure the change water roughly matches the temp of the tank and the salt is well dissolved.
That lighting is a little low for a 90-gallon reef tank if he's going to grow any serious corals. Depending on what type of corals he had, they probably died from a combination of low light, high nitrates, too much algae growth (algae smothered them), and too high/low temperatures. Corals are sensitive.
Anyway, at the very least I'd change out all of those bulbs. Even after 1 year bulbs have usually shifted in their color spectrum. When I had a salt tank I would notice that at about the year mark I would start getting growth of red algae in the tank. As soon as I switched out the bulbs it would be gone.
What kind of water flow does he have in there? Corals like/need a lot of water flow.
Does he still have rock/sand in the tank?
What's in his sump/refugium? Does it have a light on it? Does it have a protein skimmer? Any kind of chemical/mechanical filtration?