Behind My Back

We have been best friends for many yrs. We each run small firms and help each other out when we can. I have a female admin who looks after much of my work and she knows my friend and his firm. I have often wondered if I should try to date
her but she did not seem the type to date a married man . My best friend just told me that he has been dating her and having sex with her for a couple of months. He is also married. I am hurt. Do I tell her that I know?
I know it is no big deal but it hurts. It does not pay to be good