Hot Ultrapac sprayed on, leave for 15 minutes, extract with hot water.
Possibly repeat but don't forget your final pass with your prefered acid rinse to correct the ph.
It's worked for me in the past but then it all depends on what the builders did and then what the owner did. Don't always accept their first answer as they know that they don't want to seem to have made the problem worse, which she has.
If I'd been called out to it straight away then I would have put some form of absorbant powder down, even talc powder if it comes to it just to absorb the greasy carbon, vacuum that up carefully (no rubbing) and then ultrpac.
Remember they caused the problem and a magician you are not, so make that clear first.