Top layer blonde - bottom layer purple

Have you already done something? 
If her hair is already dyed and those colors are darker than the blonde she wants, you will need to lift that color somehow first, the dye alone won't do it.  Either use colorfix to lift it to yellow, or bleach it out.  (Colorfix is low in peroxide so the safest choice for the hair's health- you may need to use it more than once, depending what color the hair currently is, to get it all to yellow).  If you want the white blonde that Aguilera had, you will need to bleach it anyway to a pale yellow, then tone it with a violet toner to make it white, so you may just go straight for the bleach in this case.  You may want to put the bleach just on the roots for the first few minutes since they are dark, then pull the bleach through the rest of the hair.  Head heat will make the bleach work faster near the roots anyway, so with a little head start that should be okay.

You may want to leave out the lower layer of hair though from the bleaching step.  If you bleach it to a light color, then put burgundy, you may get burgundy or you may get a bright magenta or pink.  I'd clip off a small strand or two of hair from the lower layer and tape it to a glass plate and see what it looks like with the burgundy if you lighten the hair first, and if you don't.  She may like it better if you don't, it will be a darker color.