Usually a dead spot is one or two frets, not a whole span of notes. I’d tell him thanks for the advice but just do the setup and fret work and install my flats back onto the bass.
I was excited to try the La Bella Olinto flats but found the whole A string sounded way more dead than the other strings.
I always installed a three string retainer to get the break angle sharper on the A string, which helps combat sympathetic vibrations. Probably doesn’t do anything for sustain.
Make sure you set proper witness points at the bridge and nut, too.