At the request of a previous poster, I've gone back through this thread three times, and it seems to me that a major point has been missed. I see no indication that the girlfriend in question is submissive in nature. Several people have made suggestions for how to handle it if she's uninterested, but what if she's dominantly inclined?
A common suggestion in sex manuals of the 70s and 80s for people who wanted to try kinky stuff was that turnabout is fair play. Therefore, if he wants her to be comfortable trying bondage, or any other version of D&S, maybe he could try letting HER be the one in charge. Even if she does turn out to be a bottom, she may grow more quickly comfortable with the idea of doing it at all if she's the one with the whip hand. It can teach her the limits of what to expect from a responsible top, and therefore of what she can expect when it's her turn on the bottom.
If it turns out that she likes topping, then either the two of them find others to play with, or hope that neither minds switching.
The first two women I tried to interest in B&D both said, "Cool, as long as I'm the one tying you up." Since that didn't interest me, it never went any further, but that immediately occured to me as a possibility in this case (along with her not being interested at all, but that's been well covered).