It has been highlighted that low frequencies are very problematic. Made me think initially that the current building code is not fit for purpose, but on reflection, the costs involved in having a code that deals effectively with "everything" would escalate astronomically. Note that the code is only a minimum. If I was building new, with the knowledge and experience I have accumulated, I would drive most builders crazy with insisting on code+++ standards.

Consulting with the neighbour on a no-blame approach will likely offer the best, even if not ideal, solution. Maybe introduce them to the science outlined here by jonathan18