Difficult one that.
Probably the clean up did not include pressure washing or you would see marking on the Tarmac.
My opinion is that a spillage on Tarmac is very difficult to put right, even if you can there will be a marked difference in the area compared to the surrounding Tarmac and this may be as unsightly.
If new Tarmac then heating the area and then rolling is the best option but this is not the case here.
Have a chat with Richard Turk at Tensid UK and if the client is so annoyed with it possibly a recommended chemical & some low pressure steam.
I would always let them know there may be surface damage.
Someone else may have the magic formula - if so I would be interested as well!!
Good luck
Rob