ooh - that's a tough one. she's got hard water, maybe, leaving a calcium deposit (like the watermarks on glasswear) on the black marble.

good-quality marble has been factory-sealed for a decade or more... if so,one of the magic erasers should take the light scale off the counter. back in the old days, marble got a coat of butcher's wax to protect it...now, they sell special polishes just for marble.

I grew up with unsealed slate counters that got oiled seasonally, and waxed (by hand) twice a year to keep up the water-repellant finish...but it was worth it for the patina they took on over the years : )

cat,
The staining or discoloration for a lack of a better description, almost looks like it's in the grain. It doesn't look like white calcium buildup that's on the surface.
She used a pop-up leather cleaner and it made it look much better, if only temporarily.
The issue is definitely coming from the faucet area when hands were washed.
Thanks for the suggestion.
What about putting an oil on it? Think it would help?