All over Purple or just accent chunks of purple with the blonde? That actually looks pretty amazing... the chunks with blonde.
At home maintenance will be key, she'll need to get a colored shampoo so her purple doesn't fade as quickly, and the purple will have to be done about every 7-10 shampoos. It sloughs pretty quick when done over a full head of blonde.
If she could be "guided" into some accent pieces, you can foil the color in small sections then hit the foil (must be foil) with a flat iron enough to make the foil hot to the touch, but not sizzle, then the purple doesn't fade as quickly, bleed onto the other hair as much, or rub off on pillowslips, clothing etc. This works for any true semiperm fashion shade. Be careful not to over heat, let the foil cool completely, rinse with cool water. The heat from the iron helps open the cuticle, the swelling allows penetration of more color molecules, the complete cool down closes the cuticle and because the hair is enclosed in the color, the outer layers of cuticle are more deeply stained. The cool rinse finishes closing the cuticle and removes the excess that couldn't be absorbed. For a really vibrant, long lasting effect, repeat the procedure.