This is a very tough call because it depends on what kind of damage was done and the general health of the afflicted. Some people may not get their memory back for up to a year and some never get it back.
BUT,there are avenues for treatment. Sometimes, brain cells are only partially damaged. If the patient is given treatment in a hyperbaric oxygen chamber, it can revive those damaged cells sometimes. Other patients have had success with a modified diet and stimulation. It has been discovered in recent years that thinking, at any age, develops new synapses, the fibers that link the brain cells, through which they communicate. Playing games like solitare or any game that stimulates memory can be helpful. Just talking with the afflicted can help. I like an electronic game called Simon. It's easy to use and has different levels of difficulty. The person can start slow and work their way up.
I also suggest that you look up the web site for the Brain Injury Association. They can be very helpful.