If you're lens goes to 135mm then you've got a great focal length for taking portraits; you'll want to stay in the 50-135mm range. Go any wider and you'll put her out of proportion. If you're feeling like you need to spend money on extra glass I completely recommend just renting something, can get the 70-200 F4L IS for 4 days for $65 from lensrentals.com (used them several times, fast server, excellent support,
Rental Link)
As far as taking the picture, try to go in the evening. You'll get a warmer, softer light and can fare pretty well without any kind of flash/strobe/reflector/other modifier. Light, no matter where it comes from, is light. Also, once the sun is beginning to set she'll be able to face right into the sun and not have to squint, and you'll have a lovely fill light on her face.
If you're out shooting and it's still pretty bright, put her back directly to the sun, have her nose pointing at her shadow. By doing this you'll prevent any light leaking onto her face causing blown highlights and funny light shapes. You'll also need to A: Bump up your exposure compensation, or B: Use your flash for fill flash. One way or the other, the sky will be much brighter than her, and you need to overcome that and properly expose her. Once again, I recommend waiting until the evening when the colors are warmer.
If you're feeling a little adventurous and can pull your car out on to the beach, wait until after the sun has set and the sky is starting to turn a dark blue. Turn your camera's white balance setting to tungsten, put her back to the east and point your car headlights on to her. Your car headlights are most likely a yellow bulb (tungsten) and your camera WB set to tungsten will give you a nice white balance with her, BUT, as a very nice bonus, it will really deepen the blue skies. If you happen to have a nice bright yellow-light Flashlight or one of those spotlights those will work better as it won't spill the light around so much.
Top and bottom photos done with a Surefire Halogen Flashlight, middle with a $30 battery spotlight from Target.