The B-nice would work on rails, but the B-street would obviously be better.
Another great board to check out is the Capita Space Metal Fantasy. A very soft board, but still a lot of fun. It would rock the rails, but can still hold up all mountain. It is also quite a bit cheaper than the Gnu boards, so she can upgrade guilt-free when she's ready.
I know very little about Forum's line-up, but the Sauce did well at Good Wood last year....It's a great price as well.
And I'll plug the Pandora again - sick, sick board. Would be great on rails, but also kills it all-mountain.
If you like Gnu, check out the Roxy Ollie Pop. Very similar to the B-street, but cheaper.
If you haven't already done so, check out the board reviews at shayboarder.com - they are excellent and very detailed.