In your initial consultation, you establish what look the client wants (and doesn’t want) and then YOU choose the best method to achieve that, given her natural hair colour, any existing tint, hair type etc.
I had it done when I was young and stupid. My hair was very long and it was murder to remove the cap.... I would sooner have a baby ha ha.
Short hair I am sure is ok as the above posts have suggested, but I know it is not for the faint hearted. Steer her well away from a cap, you will be giving her your best advice when you recommend foils.