Is there a strong sense that most of the golf balls that are going into the yard/pool are being hit from the second tee box up? If so, wouldn’t simply moving that set of tee markers back onto the tips tee box alleviate a lot of the problem?
It looks like it would make the hole 15-20 yards longer, so maybe as a second step, you could grass in the first fairway bunker and extend the fairway on that side to compensate for the increased raw-distance-based difficulty. But if simply moving the tee markers around cut back on yard balls to an acceptable extent, that would be a no-cost solution (or part of a solution).
Maybe try it out for a week and ask the homeowner to report on whether there has been a substantial reduction in trouble?