I agree with TheDeSade on this, I have used something similar a different screw bit though, one thing you need to watch out for is the edges, I am surprised she hasn't cut her breasts yet.

Personally I'm more in to rope it's a lot more versatile and can easily be cut if necessary, plus tying some one up is always a great form of foreplay. But I suppose that these are quiker to put on and remove.