You need to see the hair and find out what products she’s used first and what sort of condition it’s in, porosity etc. If she is a pastel blue, I’m guessing she had to lift it to a 10 first before toning, so it could be potentially quite damaged.
If she’s used a direct dye, you need a direct dye remover. Bleach can push the molecules further into the cuticle layer and permanently stain it.
Also, you can’t use a colour reducer on direct dye colours.
I prefer to use direct dyes mixed with conditioner for pastel colours rather than tint especially if it’s dry and damaged and the client likes to chop and change.