This is pretty serious.  Not because the woman worked for a wage she agreed to, but because it throws normal employment agreements between employers and employees into chaos.  In a small organization, there is typically no "norm" that can reasonably be referred to.  The equal pay for equal work makes sense in a large corporation with many people in the same roles, but not in a small time organization with only one person in a role.  The woman has to refer to past employees to get a comparison.  With only one manager in the organization, compensation is really a negotiation between employer and employee and there must be freedom for that negotiation to occur.  If this suit is successful, small employers will be motivated to replace employees with new ones of the same sex.  The compensation of past employees no longer in the organization really has nothing to do with pay equality.  Perhaps the lower pay was due to poorer business or problems with her performance.