But Dan Flack took his role very seriously - and has given Robert one of his own kidneys so that he can get married and also be a father.
The wedding is due to take place at the end of May - and can only go ahead because Robert's life has now been saved by the transplant.
And it will also allow him to see his newborn baby, when his fiancée Louise Clough gives birth two weeks before their wedding.
Robert, 27, a security guard, says: "Dan and I have been best friends since childhood and he really is the greatest friend I could ask for.
He was my natural choice to be a best man - but even I didn't expect him to take his duty so seriously that he would give me his kidney so that I could get married and be a father. It is the greatest gift that anyone could ever give, and I can never thank him enough.
Robert, who lives in Basingstoke, was diagnosed with kidney failure at the age of three months after he suffered feeding problems at birth.
"We were told that Robert would need a kidney transplant one day, which was a shock," says his mum Rosanna, 58, a logistics co-coordinator. "We'd never imagined that it would be anything as serious as kidney failure."
Robert's condition was managed initially by medication and when he was a child, he met Dan, also now 27. The two became firm friends and when Robert had his first kidney transplant at the age of eight, with a kidney donated to him by his father, Dan visited him in hospital.
"Dan and Robert have been firm friends ever since being young, and Dan has always known that Robert has had kidney problems," says Rosanna.
"When he had his first transplant, Dan came to visit him in hospital. Robert's father had donated him one of his kidneys for his first transplant, and he made a good recovery.
"After his transplant, the boys went to secondary school together and they are both sports mad. They always spent every spare moment they had together. Once Robert had had his transplant, it gave him a new lease of life."
In March 2009, Robert started to feel tired and exhausted, and tests showed that his donated kidney had started to fail.
"That was devastating for the family as his father had donated it to him, and now it was failing," says Rosanna. "Time was once again running out for Rob, as he desperately needed another kidney transplant.
Robert was devastated as he thought his dad's kidney was going to last forever.
Robert had to go for dialysis treatment three times a week to keep him alive, where he was hooked up to a machine to clean his blood, to do the job of his failing kidney. And he was once again put on the transplant list.
He had asked Dan to be his best man for his planned wedding to Louise, but then Dan came forward with a remarkable proposal. He offered to donate one of his own kidneys to Robert.
"Robert had asked Dan to be his best man, and when Dan came forward this offer, we were just so moved," says Rosanna. "We couldn't believe that he had offered his own kidney to help Robert.
"But they have always been so close, Dan just wanted to help Robert. He wanted to save his life, and help him still get married and have his baby. It was the ultimate sacrifice to help Robert have his life back."
But they came up with a pioneering solution called an ABOi procedure, which involves using drugs to alter the immune system to allow transplants with incompatible blood groups to take place. The procedure is only performed at three UK hospitals.
The transplant took place in November at the Queen Alexandra Hospital in Portsmouth, after the procedure had been carried out at that same hospital.
"We were both very nervous before the transplant took place and hugged each other before we went down for our operations," explains Robert.
"Dan went down first and then I was wheeled down to the operating theatre a few hours later."
The transplant was a success and both were allowed home from hospital after a week.
"It was very emotional when they both met up for the first time after the transplant. Robert is so grateful for what Dan has done for him," says Rosanna.
The transplant means that Robert's wedding can now go ahead as planned in May and he also is due to be a father two weeks before:
Dan has given me my life back and has given me the opportunity to get married and be a dad. He really is the best friend I could ask for. And he certainly is the best man for the job.
Now that's what we call friendship! Good luck Robert!
More on Parentdish: Five-year-old boy gives four people the gift of life after his death