Boy's Death Allows Girl Diagnosed With Cystic Fibrosis to Live

thanksgiving, two families and one gift unlike any other. Here's "nightline" anchor, sijts ya McFatden. Reporter: Two years ago this girl struggled for every breath she took. At age two she was... See More

thanksgiving, two families and one gift unlike any other. Here's "nightline" anchor, sijts ya McFatden. Reporter: Two years ago this girl struggled for every breath she took. At age two she was diagnosed with cystic fibrosis. My lung capacity by the time I was 18, 19 years old was to the point where they were telling me that I was going to need a lung transplant in two years. Reporter: Her mother knew without the transplant lindsey wouldn't see another birthday. I would walk to work crying, knowing that my daughter was dying before my eyes and there wasn't a thing I could do about it. Reporter: Then on the morning of april 4, 2011 -- they called. They said they have a potential match. Hopefully it's for real. We're a go, yep. We love you. Love you, too. I'll be fine. Reporter: Tense moments, but in the end -- all good, all good. Reporter: Whose death allowed her to live? All I know is he was 18 and a male. Adrian rodriguez was a charmer, a high school senior, a beloved only son. Adrian was running to school and he slipped onhe subway platform. Reporter: As the train pulled in, he fell. One mother's loss would save another mother's child. All I could think of that day was how lucky we were and how unfortunate she was. We were just so grateful. Hi, how are you? Reporter: On the day before thanksgiving, the two families, tied forever, decided to meet. I think about him every day. There is a funny kind of bond I would think. I mean, adrian is a part of you. He's definitely part of me. He is why I'm here. Reporter: Cynthia mcfadden, abc's, new york.