Joe Browns do that slightly floaty hippy thing. Lots of cutaway and strappy things, though, but you can wear long-sleeved tops underneath, which is also dd1's preferred solution to anything too low cut.
Top Shop, if you can face it. Go early. They have loads of those long slightly smocky tops if that isn't too fashiony for your tastes. Primark is always worth a look, ditto Matalan. They had lots of tiered skirts in floral fabrics last time I looked (though dd1 has inconveniently gone off that style in favour of skinny jeans and, er, more skinny jeans). I know you said no trousers, but would you consider leggings under dresses? If she can get her head round the dress over trousers/leggings look you should be spoiled for choice.