It is amazing how much a couple of photos will aid in solving a problem.
I am guessing that this pond is 5+ years old and when it was installed this was the most likely location because it seemed more protected and provided that woodsiness that made it appear more natural. I have a customer with a pond installed in a similar location and like your friend has noticed something 'not quite right'.
With the passage of time, the surrounding trees have grown and, like my customer's pond, have almost completely shaded the entire pond.
This is the likely source of his problem. Without sunlight, photosynthesis can not take place. Most aquatic plants require full sunlight to survive, mush less thrive.
Your friend will need to do some selective pruning to allow the pond to receive more sunlight. Even the fish require a certain amount of sunlight.
The skimmer appears to be an Aquascape Mini-skim. Replacement brushes and support rod are available. Is the indicated object in the attached photo the ball for a float valve? Whatever it is, it will need to be removed to make space for the brushes.
I hope this sheds some light (pun intended) on your friend's pond problem.
"Everybody is ignorant, only on different subjects." - Will Rogers