I've shot a few, if she's never shot a nude she's unlikely to be really quick about it. She'll probably want to experiment a fair bit with the shot, how to pose you, and how to light you.
If you have the opportunity try to talk it through with her first, see if she can show you some ideas of how she'll want to pose you.
If she's under professional instruction for art or photography she should have at least some idea of the poses etc she wants.
Some other things to think of;
- Keep your robe close by.
- Get a copy of the release you're signing well beforehand (hopefully you are signing one as it protects you as well as her), specifically you may want something that can limit where and how the images can be used.
- Regardless of what you think you've agreed to do, if you feel uncomfortable pause the shoot and say so. If it doesn't get fixed, stop.
- Most people get fairly comfortable with being nude whilst the camera's clicking, but quickly uncomfortable when it's not. That's why you put the robe on while the lights are being changed etc.
- From your post this sounds on the level, that being so it's not about you being nude, it's about the body, that's it.
- Some people have different idea's of nude, many shots are about the curve of the breast, the back, the bum etc. That doesn't mean all of them are, many good photographs (imho) have been taken very close up to bits that often don't get close inspection, a nipple for example or... well, I'm sure you get the idea. Again, talking through the shoot over coffee before hand, and seeing some of the photo's that inspire her will help here.
If you're happy with how everything goes then, relax, enjoy yourself and make sure she makes you a couple prints of the shots you like.
Good Luck.