It should be fine to have one drink a month. That said, every injury is different and the way alcohol affects individuals is different, so you have to be aware of how one drink can affect you, post-injury. You're well aware that you lose brain cells during a head injury, which means the remaining cells must work harder to compensate. Alcohol can damage/impair those remaining cells which will make recovery more difficult. If you notice any decline or lost progress in your recovery, it is best to nix the drinking. Also if you are on any medications, it's best to check with your doctor first.

Sometimes having a drink or two can bring normalcy to one's life after a TBI, but realize alcohol is a powerful addictive, and often used it to self-medicate for head injuries, which is a very dangerous plan.

Hope that helps.