This is from TASC's website, a company who administers Section 125 plans.
https://www.tasconline.com/biz-resource-center/plans/section-125-cafeteria-plan/
[Note: one portion below]
Exceptions
Sole proprietors, partners in a partnerships, and more-than-2% shareholders in an S-Corporation have special considerations concerning participation in a Cafeteria Plan.
While sole proprietors cannot directly participate in the plan, they may legitimately employ their spouse and offer the spouse the benefits of the plan. In such instances, the employer must take care to ensure that the plan must be offered on a non-discriminatory basis. The employed spouse may be considered a highly-compensated employee and as such their contributions to the plan may be limited.
A partnership operates much like a sole proprietorship. While the partners cannot directly participate, they may employ a spouse who in turn may receive benefits. The highly compensated issues apply as stated above.
While all non-related employees may participate in the plan, depending upon the plan's parameters, non-discriminatory rules apply.
In S-Corporations, eligible employees who are not shareholders and who are not defined as highly compensated generally may participate to the fullest extent. Eligible employees, who are defined as highly compensated, excluding shareholders, will be subject to the non-discriminatory rules.
Special rules apply to a more-than-2% shareholder of the organization. These individuals may not participate in the plan; nor may their employee-spouse, children, parents, and grandparents. In determining the status of an individual that becomes or ceases to be a more-than-2% shareholder during the course of the S Corporation's taxable year, the individual is treated as a more-than-2% shareholder for the entire year.