DON'T RUB IT WITH A WHITEBOARD PEN!!!!
My experience shows it simply doesn't work and will only make things worse as the whiteboard pen won't come off either.
If Tcut won't shift it, it must have soaked in below the surface. If that's the case, you can't shift it. You could only abrade it away.
If it's painted, painting over it doesn't work well either. The paint can dissolve some of the ink and pull it into the newly painted surface.
My advice, as it's a potential customer, don't get involved.