Jean Skinner, from Essex, posted the image on Facebook – which showed a thumb nail with a dark stripe down the middle - explaining that her client had initially dismissed the mark as a blood blister.
However, luckily for the woman, Jean immediately knew it probably indicated melanoma - the most serious form of skin cancer – and advised her to seek urgent medical help.
Dark stripes on the nail can be a sign of a type of skin cancer that occurs under the fingernails called subungual melanoma.
She wrote that the client called her to say she had since been diagnosed with aggressive melanoma, and that it has spread to her lymph nodes.
'I had a walk-in nail client a couple weeks ago. She had a straight dark vertical stripe down her nail. She said "I need a colour dark enough to cover this stripe,” Jean posted.
'I did not want to frighten my client, but I told her she needed to see her doctor immediately,' she wrote.
'She called me today to tell me that, yes, the dark stripe indicated a very aggressive melanoma, and that it has already spread to her lymph nodes. Her prognosis is not good.'