Problems with Freestly breathing--I need suggestions

I teach lessons to a forty year old woman who is having a very hard time breathing. I have been teaching lessons for about ten years and I cannot figure out why she is having such a hard time. She seems to breathe in the correct amount of oxygen and then she slowly exhales through her nose and mouth before breathing again. She has to breathe every two strokes in order to get enough oxygen but even then after a 50 her lungs are burning. She can swim a 1650 in about 35 minutes and she can swim it without stopping but her lungs get tired before anything else. I have had her swim 25's without breathing and she can do that. I am completely stumped as to what her problem is. Any suggestions? Thanks