Is it legal for a company to pay for one employee's health insurance and not others?

There are federal rules that require an employer to treat all employees who are similar (i.e. work 30 hours a week) the same for benefits. In addition, if it is a group plan, the insurer would require the employer to enroll everyone who qualifies and to have one set of rules for who qualifies.

That said, there are ways to accomplish the goal legally. For example, there are no laws about how an employer decides how much to pay someone. The group plan might cost everyone $50 a week. The employer can pay the person enough is his/her salary to cover that, so that in effect, the person is getting the group plan for "free".

Similarly, the employer can pay the person a salary that coincidentally includes the cost of a non-group (exchange or individual) plan. All of this is perfectly legal IF the wages are taxed like every other dollar the person earns. (I suppose other employees could sue for wage discrimination, but that is an entirely different issue.)

The IRS recently published a piece (http://www.irs.gov/uac/Newsroom/Employer-Health-Care-Arrangements) clarifying that an employer cannot use untaxed dollars to buy an employee's individual health insurance plan. An employer cannot "send" his employees to the exchange, and then pay their health insurance costs with pre-tax dollars.