Legal, yes. Good idea, probably not. The visibility is considerably better in a hockey style mask. Generally speaking, you can leave it on for all plays, whereas with the old style mask it helped to rip it off for pop fouls and plays at the plate.
I realize this is an older question... But Girls Fastpitch Softball references a study done by Kettering University comparing safety aspects of hockey style masks and traditional catcher masks. I don't know that the Kettering University is still available to vistors, I have read the article and it seemed very compreshensive. My best recommendation would be to reference the article at GFS should you be interested. It's an informative read. Girls Fastpitch Softball: July 29, 2007
Try several hockey style masks and she may find one that fits her correctly. I don't know how old your girls are but if they are in between the youth and adult it is hard to get a good fit. The other thing there are restrictions for two piece gear so make sure you consult your rule book prior to showing up at the field to play.