If you bleach her hair to lighten it and put a semi permanent pastel colour on top, it will take ages to wash out and will never completely leave her hair. A compromise would be to dip dye just the ends, which can be trimmed off after the holidays. It's not that difficult to achieve at home. I would recommend Schwarzpof LIVE lightener + twist in pastel rose or lilac. This would lighten and tint the hair all in one go.
Alternatively, she could try hair chalk which is wash out, available in Boots.