Do you know where the lady had her nails done? If it was a NSS you are going to be there for a looooong time removing them!! Also I would suggest that you mention to her that her shellac may not last as long if her natural nails are in poor condition after removal.
For removal though the best way to go about it is to soak her nails in acetone, for maybe 20-30, hopefully the product should dissolve off no problem!you will need to scrape excess product off & possibly re-dip.
For a speedier removal I do it bain-Marie style (bowl of acetone in a bowl of warm water) the warmth of the water speeds up the removal process HTH X