Just remember -- there is almost no scenario that will work-out in your favor. Consider these possibilities:
1) Getting caught by the husband or the husband's PI balls-deep in teh poon.
2) Some devastated guy showing-up to your work insisting he speak with you "man-to-man".
3) Her going basket-case on you and deciding she wants to leave her husband but insisting you be there to support her.
4) You deciding you finally know her well enough to eat the nay nay the same day she and her husband have angry make-up sex.
5) Her melting down and telling all her friends about the affair. Then at every bar you frequent, you'll be known as the guy that "so-and-so cheated with".
6) Her leaving him showing-up on your doorstep at 3:30 am with a packed bag telling you she finally gathered the courage to go.
7) On your second secret hook-up, she introduces you to her four children afterward. Then they know about mom's special friend, but aren't supposed to tell dad.
....
And think about all the other messes this could create. No nay nay is worth the potential mess.