If the line is freezing off there, you probably have a dip in the line that collects fluid.
Does the line ever come back up the surface before entering the pond? If so, you might be able to install a "blow off" valve.
You turn your air pump to max rate and then open the blow off valve. The theory is that the air velocity is much higher through the full-opening valve than through the membrane bubblers. If the sizing is right, the extra flow velocity will entrain the fluid in the low spot in the line and you should either see a mist or slug flow through the valve.
The other option is to fill up an air tank with your shop compressor. Connect it to your line and give it a few shots of high pressure gas. Obviously, you must do this with the blow off valve fully open so you do NOT blow a hole in your line in an inconvenient spot underground. (Pipelines are devious that way!)
With the supply line size and pump, he won't generate enough air volume/velocity to do what you are thinking it will do on the compressor alone.
If using a portable tank, he'd have to shut off the diffuser before he tried it and it'd only work if the water was liquid, not solid.
