We bought this AllSun adapter . It was able to confirm some smaller dropline leaks, but it missed more than it found. It was completely useless at finding the larger leaks (mostly in saddles and 3/4" tees) that seem to be the bulk of our problem.
So I'm pushing him to try the hearing aid apps -- this one is expensive, but seems to have a good combination of EQ and pitch shifting that may make it possible for him.