Margaret's decision meant that Bobby, 26, received her son's heart, and is now able to live his life to the full. It has also allowed him to become a father after his fiancé gave birth to their son Charlie, 11 months.
And now after a friendship which began with a heartfelt thank you letter, Bobby and Margaret have now met each other for the first time.
Bobby, a project manager for a building company who is engaged to Rebecca Baker says: "It was so wonderful to meet Margaret. We have been talking through letters and emails, but to meet her in person and be able to thank her was so emotional.
"It is thanks to her and her son Stephen that my life was saved and we have been able have our son Charlie. It was such a brave decision to make and words can't express how grateful I feel. I was nervous about meeting her, and I do feel guilty that I lived because her son died. But she was wonderful and our friendship will just get stronger and stronger.
"It was amazing for her to meet Charlie too. Without her, we wouldn't have been able to be parents."
Bobby, who lives in Tonbridge, Kent, was born with a congenital heart defect, with only one pumping chamber. Doctors told his mother he wouldn't live past three months, but he defied the doctors' predictions:
I was born in September and my mum was told that I wouldn't live until Christmas. I didn't have an operation, but gradually my heart adapted and my one pumping chamber did the work of two.
"I was smaller and weaker than other children my age, but I went to school and did my exams and managed to live a normal life."
But then at the age of 22, Bobby started to go downhill. He had an operation to try and redirect the blood flow, but it failed.
"There was no improvement after the operation. The doctors wanted to wait for six months to see if it got better, but nothing happened. I just carried on getting worse," he explains.
In March 2010, Bobby was placed on the transplant list for a new heart. By June he was so ill that he went for a check up at the Freeman Hospital in Newcastle and he was kept in.
A donor heart became available and he was told that he could have his transplant in October that year. It had come from 26-year-old Stephen Valsechi, who lived in Belfast, who had passed away.
His organs were donated after Margaret, who lives in New York, made the brave decision.
His heart was donated to Bobby, whose life was saved by the operation. Three months later Bobby wrote a letter to Margaret, thanking her for her decision.
"I wanted to thank her for what she did, so I sent a card through the hospital for them to send on to her," says Bobby.
Margaret and her daughter Ellen then sent a reply and a friendship was struck up between them. They have since written regularly to each other.
When Bobby became a father for the first time when Rebecca gave birth to their son Charlie in October, he wrote and told Margaret, whom they now call Nanny Margaret.
"Margaret sends gifts for Charlie and we send over photos and videos so she can see how well he is doing. Charlie is only here because of what she did," says proud dad Bobby.
Bobby met Margaret and her daughter at the Westfield Health British Transplant Games for the first time recently and it was an emotional first ever meeting.
"It was wonderful to meet Margaret and Ellen. We are looking forward to spending a few days together," says Bobby.
"Stephen was a registered organ donor so we didn't hesitate to donate his organs as it is what he would have wanted," says Margaret.
Stephen helped save the lives of four people. As a family we are so thankful that Stephen's heart went to Bobby, whom we have come to love and respect so much. It was emotional meeting him, but we were really looking forward to it.
To join the donor register, visit Organdonation.nhs.uk.
Words: Lucy Laing at Worldwide Features
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