Agreed, this sounded odd to my ears...!
I presume it should be ‘she does not want a dress’....?
...and the reason they’ve written it as they’ve have is because the Gaelic is more literally formed by verb ‘to be’/‘is not’ + want(ing), ie. tha/chan eil... ag iarraidh? I think they’d be better to give a tip/explanation at this point?!