Depends tremendously on experience level, whether or not the designer can implement in code (ie, works in HTML/CSS rather than Photoshop), and other important factors.

Having recently hired both designers and engineers, I can say with confidence that the pay differential is a hell of a lot more than 7%. Double is absolutely plausible.

Could be any number of reasons for this. Salary is surprisingly rigid over time for many because people simply don't like negotiating. Women especially negotiate less than men. Maybe she had a $40k salary at her previous job and $60k looked like an amazing deal when in reality is is way below market. Who knows...