If it's a direct dye semi, you need to use a remover formulated for direct dyes. Joico do one. You could also try a clear tint plus 6%. Whatever you do, don't use bleach as this is likely to cause it to stain the cuticle layer and then you're stuffed.
If it's an oxidised colour, then a colour reducer might be better.
After removing the colour, you need to lighten the hair to pale yellow, then tone before applying a light pink, otherwise any yellow tinge will turn it to a coral pink rather than a soft baby pink.
This is definitely a case for lowering the client's expectations as getting pastel colours to look beautifully even over previously bleached/coloured hair isn't easy and really depends on the hair you're working with. How porous it is after removing the current colour, etc.