Dreaming about a coma could be the result of an injury to their head, most likely from a concussion that may have triggered it. In the dream, there is also the chance that the person in a coma has lost temporary sensations of their limbs or experienced temporary numbness all over.
The dream could also symbolize that the person has forgotten a large portion of their waking life. Be wary of dreaming about comas, and make sure you are still able to recall your memories well.