Let's break this down into pieces.
1) Is it against the law for an employer to buy an individual plan for an employee - No, there's no law that says an employer cannot do this.
2) If an employer does buy an individual plan for an employee is it tax qualified (i.e., is it exempt from income tax to the employee)? No it is not tax qualified. The value of the plan should be taxable income to the employee.
3) If you have one employee getting individual plan, does every other employee have to get that? - No. Only when a group plan (which is tax qualified because it is offered to all employees at a certain threshhold - usually 32 hours per week employment) is in place does the plan have to be offered to all employees within the coverage status.
4) When does a group policy have to be offered? currently only employers with 50 or more full time employees must offer group health insurance.
So to answer the question ultimatley - must the employer offer a group plan? No. However, if you are not on a group plan, the value of the plan purchased for you should be taxable income to you...and if it's not being reported as the same, that's a violation of ERISA and the tax code. Hope that answers your question.