Roxie,
I don't know you, and am certainly not judging you or the situation. But I agree with Soklueles. Besides the whole karma thing (an expression) where is the upside? Even if this guy ever did leave his wife, assuming that you are looking for more than just a passing fling, why would anyone want to be with someone who did that to his previous wife. Even if she is the biggest bitch on earth, he should at least be man enough, and adult enough, to handle the situation like an honest individual. That assumes she is that way, and it's not just him who's the ass. Ironically he will likely at some point determine that a woman who cheats with a married man isn't "good enough" for him. People tend to rationalize away their own deficiencies, while magnifying those of others.
Also, if it is nothing more than a fling to you, then why screw around with someone elses life? This guy made a commitment to someone that he's chosen to blatantly disregard for his own selfishness. There is no shortage of unattached guys willing to fill that role, and possibly grow into more.
Just my two cents. I'd give the same advice to my sisters. I just don't see this ending well for you.