If the employer provides housing to the employee, the employer has to identify whether it is taxable or not. If it is - the amounts would be added to the taxable income on your W2. All the withholding and FICA tax calculations will be performed based on that taxable income. If the employer fails to do that, and you get audited, you can be left on the hook for the unpaid taxes on the unreported income.
In some cases, employee housing is a non-taxable fringe benefit, in others it is taxable. Your tax adviser will help identify which case applies to you.
After you added in a comment that you're trying to see if you should be asking your boss to pay your personal expenses vs. giving you a raise - as I said in the comments, your personal expenses are not deductible neither for you nor for anyone else. If your boss pays your rent instead of a raise - its taxable income for you.