Some of the basics:
Foot/Hand warmers - the heat packets can be found at any store. Always advisable for when you're sitting in the blind. It helps!
Camo/Waders - Cabela's just came out with a women's wader. See NorCal Cazadora's blog for more information (and general clothing tips and tricks). I use the men's RedHead waders and wear 2 pairs of extra socks to fill up the shoe (I'm a size 5 in men's but the wader part didn't fit, so went up to a 6). A lot of it is trial and error for what fits.
Boomsticks- A lot of women start with 20g or youth models, I shoot a 12 (started with a Citori over/under, just moved up to the Benelli M2). I'm a bit on the curvacious side with a bigger frame so the 12g works just fine for me. Make sure she heads out to the shooting range before hand to find a gun that fits her (if she doesn't already have one).
Ducks - Teal are quick! I usually pass on them as a new hunter (I'd rather keep my cripples down to a minimum). Mallards and sprigs are pretty nice targets. I don't even bother with the geese (one day). It's about comfort level for each person. I highly recommend shooting clays (if there's a course near you, that's been extremely helpful for me to judge distance/speed) in the off-season to help with accuracy.
Fun - Enjoy the time. There's a lot more women hunters out there than meets the eye. While we don't have the same numbers as men, we are out there. There's a lot of information here in these boards, so I highly recommend that your girlfriend reads through. I've found it extremely helpful!
"The ability to quote is a serviceable substitute for wit." - W. Somerset Maugham