See below... Support ur friend lovingly. Do not shame him/her; if u're somewhere together and he/she needs to suddenly leave, accomodate him/her without asking questions... Do not assume the therapist role with your friend, though... Ur role as a friend is priceless! Encourage him/her to get in treatment...Wishing u both well!
Care and treatment. Be supportive. Understand that what may be easy for most people can be very difficult for someone who suffers from anxiety. Be positive and hopeful. Encouragement without pressure is helpful. Consideration, tenderness, care and love are especially important. Help your friend find a psychologist specialized in treatment of panic disorder.