I chose 38mm sport, but I am 5'4" and wanted something small and light weight. If she wants 42mm then that's what you should get. She knows what works for her.
There's no way to tell which to get from what other women say. She really needs to try them on. Just pick one and if it doesn't fit, return it for a different size.
I went with the 38 and it was an excellent decision. It's not too large or too small. Most of the time I'm looking at black glass - didn't want it to be too big.