Truly, depending on the person, the first bike can truly be critical. If this person is petite, cautious, and not really sure about this sand stuff, you need to choose a bike that is easy to start, ride, reliable, smooth linear and adequate power, and very comfortable. Sounds like a lot of stuff to think about, but it could make the difference between the person having an enjoyable experience and making many more trips with you, or hating it, and refusing to ever go again.
With those considerations, I'd seriously consider the Kawazuki twins. Until I got my wife on her kfx400, she was absolutely hating the whole thing. She's now just about to the point where she could hang with the group, and enjoying herself and the challenge of learning something new and exciting. When she first started, it seemed like she used reverse more than the forward gears, so don't discount that neat little feature. The great thing about the kawazuki's is that as she learns, it's going to take a long time (if ever) to outgrow the bike, as in stock form, with just paddle tires, I can take my suzuki 400 anywhere in the dunes. That saves money in the long run. Also, should things not work out, you'd have a nice bike for yourself to ride..