You have to show it as a bartered income if he did something for you in exchange for the "free" rent. Like he maintained the property or did any other "free" service for you. What that value of his work is would be based on fair market value for that service.
If you simply gave it away, the only concern is if you did this in a manor that created a business loss for too long a period (three years?) as it could be construed as tax evasion.
Usually if this was done in good faith for a short term, you do not have to do anything.
