That depends, and you have not given us enough information to say for certain.
Despite a persistant and widespread belief, the law does NOT say that if one employee gets a benefit, all employees have to get it. It IS legal for an employer to set up "classes" of employees, with only some of those classes being eligible for benefits.
It IS legal for an employer to offer insurance to full time employees but not part time, to corporate employees but not employees in the field offices, to managment and above but not below, to office but not shop, to employees in certain departments only, or any other grouping that does not violate Title VII and related laws (race, religion, gender etc.).
Having established which are eligible classes, however, all employees in the eligible class must be offered insurance.
So you would have to tell us why the employee is being denied benefits before we could say if it is legal or not.