I personally wouldn't say a word, unless its a best friend or someone REALLY close too me. I would also have to make damn sure that I'm positive they are cheating. If It was someone that I just new I wouldn't say anything. My sister was cheating on her Husband and I new It but I wasn't going to tell him, thats just wrong.
And then comes the exception to my rule..I will also not tell If I'm involved (meaning the person is cheating with me..God I'm so bad

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