At this point I would probably restart everything, gravel included. Clean up the tank etc. There must be hundred of snails eggs everywhere in the tank/filtration, plants. You could help him to cycle faster by providing him some of your bacterial benefits coming from your filtration. That would fix the snail issue otherwise anything that inside the tank right now, if he put it back again, the problem will come back. To have green water like this there must be a strong source of light that way to strong or light is on for to long.
So here are the steps I would do
1-Transfer all fishes into a quarantine tank, until they are healthy or there no any sign of fungus
2-Clean up the tank
3-Clean up the filtration, and let it dry under sun
Once the tank is clean and dry, add new gravel(maybe he could keep the old one, but I would let it dry pretty well.)
If he doesn't mind about the snail, i would just perform water changes until the water clear out and cut off the light as much as possible.
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Last edited by blesser13; 07-04-2015 at 05:54 AM.
Reason: added comments