what's causing the green water?

[Van down by the river]

UV will reduce or remove the green water, and I would test your tap water if the green is persistent.
I also think it's most important to look at the cause, Is it the water? Photoperiod (how long are the lights on), Is the tank new? Does he have the right lights or are they $2 homedepot cool whites! Is he overfeeding? Is it getting allot of direct/indirect sunlight? Any of these, or a combination could be the cause. Has he made any recent changes? Through process of elimination you may solve the root of the problem.
If you do use a UV, monitor your fish as you may need to add temporary airstones or do additional water changes as the possible initial mass die off of the algae could cause low oxygen, killing your fish.