Just to be clear, I was merely offering her another suggestion as to how to achieve the look as asked. I would in no way take away from the original artist's talent for this gorgeous cake, which can be see here: http://www.cakecoquette.com/gallery/.
Either way, you may just have to tell the customer that you will not be able to give them the cake they deserve in the amount of time provide and offer another suggestion at a small discount.
As for straight lines if you have to pipe directly on the cake. You can use guides, such as lightly scoring your design on the cake. Just remember that when using black piping on a white cake, you will have to take extra care not to make large mistakes that will need to be move. Black stains quick and is very unforgiving, it will show a major flaw if trying to move a line. Make sure you fondant is dry, no condensation, work in a cool place to keep down the humidity and use royal icing. If you make a mistake, let it dry before removing. You make be able to remove some of the black residue with a q-tip, clean small paint brush and grain alcohol or vodka. Again, good luck and happy caking. I really hope it turns out Great! Let us know.