is the tank in direct sunlight? if it is, shift it out of the sunlight. or they can use a UV filter which will kill floating algae such as greenwater etc. another alternative is load the tank with plants which will use up all the nutrients in the tank, which in turn will starve the algae. I'm not sure about this last one, as it works for my tank, but it may not work in theirs. someone else here should know.
Can also try fully covering the tank so no light gets in, for a few days. this won't harm the fish. can someone confirm this, as i'm sure i read this method somewhere on this forum.