Flatwound Strings have a more prominant 'Fundamental' than Rounds,
and thus when 'seen' by the narrower magnetic field of a Single Coil rather than the parallel coils of a conventional humbucker give a more articulate note, than the more smeared mix of Rounds seem by two rows or magnets/windings.
2 of my Passivated Ibbys, an SR300 with Maida Vale Stack Coil pickups and a SR400 with Dimarzio Twin Coil Jazz pickups, along with the 38mm Nut and sleek neck give a great Jazz Bass-esq playing experience with a lovely worn pebble body and without that funny lump.
My 2 Actual Jazz Bodied Basses are PJ's.
Recommend you (your friend...) give Flats a whirl, and Adagio Flats at £18ish are a real good toe-in-the-water place to start. I have em on a few...