Anna Bianconi-Moore, a nurse in the dermatology department of Addenbrooke's Hospital in Cambridge, was watching the soap last autumn when she spotted a mole on Nicholls's shoulder.
"I noticed it was irregular in shape and had at least three different colours that I could distinguish by standing close to the television," Bianconi-Moore told The Mail on Sunday.
"These are two of the red-flag signals that distinguish the most deadly form of skin cancer - malignant melanoma.
"I was obviously incredibly worried for Sue, and felt I needed to do something."
The 55-year-old immediately went online, found an email address and contacted the show.
"I wrote that I had observed a sinister-looking lesion and suggested that Sue should see a specialist, sooner rather than later, as it may require urgent attention," said the nurse.
"I didn't want to be perceived as a deranged fan but I felt a moral obligation. If you saw a person about to step out in front of a bus, you would pull them back and that is how I felt."
Bianconi-Moore was told that her message had been passed to the soap's in-house medical team, who measured the actress's mole before checking a few months later to see if it had grown.
When the mole was found to be a quarter of an inch larger, Nicholls - who plays Audrey Roberts - was referred to a skin cancer specialist, who decided that it should be removed.
A week after having the procedure, Nicholls's surgeon contacted her to say that the mole had indeed turned into melanoma.
ITV eventually contacted Bianconi-Moore late last year, updated her on the situation and invited her to the Coronation Street set to meet Nicholls.
"When I found out what had happened to Sue, I was sorry I had been right, but there was also an element of relief," she said.
"I'm so glad I was able to do something, and she is now well."
Nicholls said: "It was so important to me to meet Anna, and thank her. She is such a kind, conscientious and clever lady.
"I wouldn't have done anything about my mole had she not got in touch. It was just a freckle, and I'd had it forever." Get the Inside Soap magazine on your iPhone or iPad