Why does she want to dye her hair? Is it to enhance her natural appearance or does her brown hair look somehow odd that she would be stared at? Or is it to show off? Impress some man? Is she just curious? IF she does it, try a non-permanent dye that washes out. Oranges, reds, blondes tend to all look brassy and fake. She may find herself wearing scarves and hats for several months if a permanent dye turns out badly. Why not condition, shine and style the hair God gave her? Dyes contain ammonia and peroxide and are very hard on the hair and scalp. I doubt it's a mortal sin unless deceit or lust is the motive, but I can think of better uses for time and money. She'll look better if she uses the time it would take to pray an extra Rosary.