Might be very naive of me to say, but I would outright say she worked as "x" on her resume and to prospective employers. If she's actively fulfilling all the job responsibilities of that higher position, whose to say she's not a, for example, Director of Finance vs. Financial Analyst? When getting interviewed and questioned on this, she should be able to easily explain all the job responsibilities that made her a "Director of Finance."
Tell her to keep records of the job posting, and its relative job responsibilities, to allow her the chance to show that she indeed was that position without officially being named that by the company/within HR (in case prospective employers call her previous employer).
In terms of the immediate question at hand, I would lean towards the employer screwing her over as she's helping their bottom line. A little leverage would go a long way!
GL