What's wrong with just saying, "I know you're excited about this bike, but I don't think it's the right bike for your style of riding..." and then listing the reasons why (clipless, carbon fiber, she's shorter than you, etc.)?

You could explain to her that bike fit is an important part of any bike purchase, and you'd like to help her find a bike that would be perfect for *her*.

Be bold, but considerate of her feelings. I have a feeling she just wants to be like you, and just doesn't know what she doesn't know about cycling.

Karen