Green cloudy water is a manifestation of an algae bloom. Algae aren't always that prominently visible, but they are always present in a living aquarium. Aquarium of your friend is having recurring algae blooms because the aquarium is trying to save itself from "crashing" - in other words, from poisoning its inhabitants with excessive concentration of nutritious pollutants present in the water; mostly nitrates, probably also phosphates. Please ask your friend whether she could give us a reading of nitrate (NO3‾) concentration, it would help a lot. Partial water change is a good immediate response, but it's tedious and doesn't solve the original problem causing these recurring algae blooms.
My suggestion for solving this is to significantly increase the density of live plants in the aquarium - optimally with quickly growing plants, such as hornwort, duckweed and pondweed. Algae are simple, plant-like forms of life and they compete for nutrients with actual plants. Increasing live plant density will increase competition for these nutrients and increase the rate in which they are absorbed from water column; this will slow down the growth of algae. Fast growing plants are the best for this, because the growth speed of a plant is directly related to the amount of nutrients it absorbs. Some plants, such as hornwort, are also specifically releasing substances which inhibit growth of certain forms of algae.
What is more, it could be helpful to reduce the amount of food being fed to the fish. I don't know whether it is a problem in case of your friend, but it is still worth checking out. Almost all people that I encounter, and I am also sometimes guilty of this, are actually feeding way more food than the fish actually need. Even if the aquarium is not overcrowded, by overfeeding it is still possible to strain the bioload as if it was actually overcrowded; it is because aquarium is a closed system and all the food introduced to it, no matter if eaten by fish or by bacteria, is ultimately going to be metabolized and partly converted to nutritious pollutants.
Also, it could help to decrease the duty cycle of the lighting being set ON and make sure the aquarium isn't directly exposed to sunlight - even a short daily exposure to sunlight could enormously increase the algae growth.