You can put a tiny amount of vasaline around the fingers using an orangewood stick, so if you do get any on the skin it comes off easily, I do this when using the black pigment as its hell to get off
remember you only need to work with tiny amounts of pigment too. She could possibly try using a little lemon juice to remove it or sterident tablets that are used to clean false teeth in some water and soak them for 5 mins, ive suggested this to clients that have stained there natural nails under enhancements and its worked doing no harm