Not pure enough water is most likely the culprit for the algae bloom. Freshwater tanks have a different biosphere and can handle more iron, phosphates, and other trace elements that cause a green algae bloom like your friend is having. It's like comparing apples to oranges there. It will take time, a lot of water changes with good pure water, and probably running something like carbon to take out the trace elements and clear up the water. The algae bloom will persist until it starves...