Most studios are not very formal. Many people wear Lulu's and all types of things. It is considered proper to wear dresses for ballroom though. And those can be literally anything. Especially starting out. I usually look for things that are very lightweight no matter the season because I sweat a lot when I dance. For Latin n  rhythm dances they are usually shorter skirts so that you can at least see the knees. For ballroom Standard or smooth they can be long skirts anywhere from the knees all the way down to your ankles. I personally do wear dresses always. Many people do not. I suggest she just try some classes first to see if she's new. No one will be expecting any sort of big dress code from her for the clothes. More important are that she has proper fitting ballroom shoes.