I have a solution that worked great for me.
Made a trip to my local craft/hobby store and found a saw with a blade .022 wide. Found polystyrene strips .020 wide.
Chased the fret lines with the saw - cut the polystyrene into small blocks, slightly larger than the fret slots and super-glued it in the slits. Trimmed the excess with a razor blade, than sanded lightly. The styrene sands more easily than the fretboard, so you won't change the radius of the fretboard.
I added one coat of tung oil over the fretboard, which made it look nice.
If the owner can live with visible fret lines, the plastic styrene is non-compressible and EASY to work with, and you can invest minimal time in a good result.
The result: