Nothing, really. Salt will stay in the water, even if water evaporates the salinity will go up. Most likely thing IMO is he never mixed the correct salinity in the first place, What is he using to test? He should use a refractometer, if he is using a hydrometer they are not only notoriously inaccurate, but also are affected by temperature so you will get a different reading if the water temp has changed.
My thoughts on his start would be that the fish may be OK, though the clown pair may eventually not work out in a tall 15g (though I have seen people with pairs in smaller). His lights look very white-toned, which will be good for growing algae but not for corals. If he is using the light that came with the aqueon 15 gallon, that light will end up giving him algae issues. For such a new tank, can already see some GHA growing which will be a nightmare if left unchecked. Did he use live rock, or dry rock? If it was live, he may be fine skipping the cycle, if it was dry his tank is probably not ready for fish.
eon aquatics
since you don't have experience with marine setups at all, please refrain from giving random or "google copy-paste" advice. Using Zeolite in a marine setup can strip things from the water- it can have use, reducing nitrate and phosphate, but sometimes in a reef you want those things, so it should not be used normally as a substrate.
