Is It Safe To Fly With Potential Concussion?

My daughter was recently rear ended by another driver and she is due to fly over seas. She saw a Dr. at a critical care clinic who suggested she might want to get a CAT scan done to rule out bleeding in the brain. What are the risks of flying with a potential concussion. She is a 27 year old with no previous medical history. She is 5 foot nine and about 150 lbs. She is in top physical shape and is a vegetarian.