I have been cheated on before. No one told me but I had a women's instinct and did all I could to find out the truth.
Also, I firmly believe that everyone has the right to know what their partner is up to, especially with today's diseases and such.
With that said, telling her would depend on your level of friendship. My best friend and I have discussed that we would want the other to tell whatever knowledge they had. I have other friends that I feel it's not my place to tell unless they ask. You also mention that he is your husband's best friend. That friendship needs to be honored too. If it was me, I might say something to him because he needs to tell her or stop. Cheating is just wrong. It's hurtful and dangerous. But he is your husband's friend so maybe let your husband determine if you should say anything or not.
Also, I firmly believe that everyone has the right to know what their partner is up to, especially with today's diseases and such.
With that said, telling her would depend on your level of friendship. My best friend and I have discussed that we would want the other to tell whatever knowledge they had. I have other friends that I feel it's not my place to tell unless they ask. You also mention that he is your husband's best friend. That friendship needs to be honored too. If it was me, I might say something to him because he needs to tell her or stop. Cheating is just wrong. It's hurtful and dangerous. But he is your husband's friend so maybe let your husband determine if you should say anything or not.