So many people who don’t believe in the pay gap argue that you’ll get rewarded based on your performance. In this case, the biggest thing this person has going for her is her performance record. So, ask the company if they give commissions for bringing in new business. If not, why not? Look at other firms and industries (law firms, consulting firms, tech sales) to see what their compensation packages look like. Since she knows what she should be making, set that as her salary benchmark and work backwards to see what kind of sales commission that represents. Encourage the company to make that an established policy, so that everyone is working under the same incentive structure.