Personally I wouldn't go with a pen, its too personal/individual, I write loads these days(degree, programming and computing... writing both is a pain in the ass and long winded when done by hand), the wrong type of pen just leaves you with painful claw hands. I wouldn't drop £50 on a pen that might make her hand hurt.
Get a bunch of various cheapo £2 pens and a £50 voucher, let her play around with them, then when she picks a comfortable one she can go pick a "fancy" pen that is as close to the cheap pen as she can get.
The other way to go, instead or aswell, some personalised stationary, what kind of writer is she, letters to friends... then a bunch of personalised letter paper/envelopes/boxset, if she's a creative writer, some kinda on the move leather bound monogrammed notebook that she can put idea's in, or a bigger at home a4 leather bound pad thing, personalised paper is a bit pointless.. though if she actually does stuff like write books for submitting, a few sheets of personalised stuff for the top page might work.
Maybe a fancy old typewriter, a lot of writers like the old school feel of a noisy type writer, also its easier to write properly on a typewriter than an actual computer.... a full on computer = massive distraction, type writer = writing only, better focus.
Naked lady pen (the type where you turn it upside down and clothes are removed.
This was my first idea......
