Bubblers are fine for smaller volumes, but they don't really keep it clean. I have a smaller pond next to the pool with a waterfall that creates bubbles and it's still green. I don't mind it there since the goldfish eat the algae. He might be able to do something that pushes more air, but at what cost?
The place that I referred to is a farm pond, so he must be doing something different than what you describe.