It might be worth trying to sell grout colouring, as you'll struggle to get it back to a uniform white. If you use Aquamix Grout Colourant then you can sell it as making the floor uniform all over, with the benefit of giving it a waterproof epoxy seal. I'd actually try to convince her to colour it grey, as white grout in a kitchen is always going to take a hiding and need a lot of maintenance.