Level 5 - 6 bleach bath / straight bleach for lavender

It won't be light enough for a pastel like lavender--it needs to be very pale, inside-of-a-banana yellow (a level 10), preferably toned to white/silver for pastels to take well.

Thanks intem, i meant if one full bleach session would be enough or of i needed a color rekover plus bleach.... It shows it was like 4 am when i wrote it!

Possibly, it depends on the condition and resistance of her hair.  If she got 2-3 levels of lift the last time she bleached, it's likely she'll get another 2-3, which would take her somewhere between a 7 and a 9 ... if she gets to a level 9, you may be able to get away with a stronger pastel shade; if she gets to a level 7, you will probably have to do a bleach bath or two for the last little lift.

I don't think you'll need a color remover at all. If she used a blonde box dye to lighten her hair the pigment deposited from it was much lighter than what her level turned out to be so I don't think it will cause any interference with bleaching.

If she feels like investing a little money, Matrix Light Master is a wonderful bleach and lifts extremely well. With a 30 vol bleach bath (so 15 vol) I am able to lift very resistant level 6 hair to a 10 in 25 minutes, and with level 5 hair I usually use 20 vol for 30 minutes and it's pale yellow.