Employers can't discriminate in employment on the basis of race, gender, national origin, disability, age over 40, color, creed, etc. But I know of no rule that requires employers to treat employees equally or fairly. Assuming a union is not involved, employers are free to do dumb things that destroy employee morale, like hiring new employees at higher wages than the employees that are already there.
Not legal advice as I don't practice law in Washington. It's just my two cents. If you need legal advice, please consult a lawyer who holds Washington licensure. That's not me.
Mr. Taylor is correct. Washington is an "at-will" employment state, so as long as there is no union contract or discrimination, and the employer is paying at least minimum wage ($9.04/hr this year, adjusted annually for inflation), employers and employees are free to negotiate the terms of employment. If the employer refuses to increase her pay, her remedy, unfortunately, is to find a different employer. This publication has more information about workers' rights in this state: http://www.washingtonlawhelp.org/documents/1493012908EN.pdf?stateabbrev=/WA/