Let me see if I get this straight: your GF wants to ride a bicycle to work in the winter, and yet she's concerned about appearing fashionable?
Riding a bike -- period -- isn't fashionable. Ain't gonna happen, no how, no way, never. Pearl Izumi could sponsor the next Victoria's Secret Runway Model Show on Pay-Per-View and it still would never be considered fashionable.
When she's on the bike, she's not fashionable. Even if she were decked out from head to toe in the latest haute couture straight outta this month's Vogue, the moment she straddles the bike all cred is lost, all fashionableness is negated.
There is absolutely nothing she can do to overcome this condition. If she wants to ride a bike in Winter -- or in Spring, Summer, or Fall -- she will not appear fashionable. Sorry.
Now, if she wants to appear fashionable once she gets off the bike, she could either get decked out from head to toe in the latest haute couture straight outta this month's Vogue...and be cold, wet, miserable, and out >$8,000 for the wardrobe...or she could dress practically (see everyone else's suggestions in this thread) and carry her Fashionable Wear™ in a backpack, to change into once she dismounts.
So long as she is on the bike, she will be unfashionable. We all are. Remember that.