Depending on her size, guys stuff would work. Phalanx makes women's rashguards and I haven't tried them yet but hear they are good. If she is tiny then the women's stuff would work. I've found most seem to be cut for juniors or girls without boobs/ass so if she has either then they may not be comfortable.
I wear an xsmall (28) in shorts over cotton leggings from H&M. For tops, go to your local sporting goods store and get some women's long sleeve compression tops (or short if she prefers). I know a lot of girls who just wear an under armour top and spandex capris or leggings meant for running too. It's all about what she feels comfortable in but to start, getting things from Under Armour or Nike will work best because it is most affordable. Have her try on rashguards of teammates about her size to see how men's stuff fits before purchasing too.