Andy have you tried an ink or colour stain remover?
I've found zip-it from one step excellent on many different kinds of ink marks.
I'd try that and possibly prochems stain pro which I've also had good results with.
If they dont work and it is a dye and the colour has set at least you've tried and can charge them accordingly for your effort
I would pre warn them that its unlikely to come out but you'll give it your best shot.
Good luck!
All the best,
Jason.