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 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.