It's a little hard to know how to answer without more details, and I understand why you wouldn't want to go there. I'm wondering if perhaps the woman had already committed adultery against her husband with the 2nd man that she eventually married, or was at least planning on it to the point that she had committed adultery with him "in her heart" as Jesus spoke of in the sermon on the mount. IF this were the case (as is very common today) then she is an adulteress and the first husband is free to leave her (or let her leave him) and is free to re-marry.
I had a lengthy discussion about this kind of thing with a pastor once. We really didn't come to any kind of definite conclusion, but we were discussing when a divorce is even recognized in God's eyes. In other words, if two people made a vow of marriage before God and they divorce without a Biblical reason (adultery or fornication), does God even recognize that divorce? I have to really question that God would honor a piece of paper issued by a human government that says they are divorced when there was no grounds for divorce authorized by God (no one had committed adultery or fornication). If God doesn't recognize the divorce in the case you are talking about, even if there wasn't adultery when the woman left her husband, there is now since she is pretty obviously sleeping with a new man (if indeed God didn't recognize her divorce from her first husband). And if this IS the way God sees it, then she has committed adultery against her first husband even if it occurred after getting secular divorce papers, and he is free to go on and marry another woman. Of course, I'm also assuming the first husband never agreed to the divorce and hopefully refused to sign any papers saying he would agree to it. If he did agree to it, well then all bets are off unless she had actually committed adultery before their secular divorce.
However, like I said, the pastor and I never really came to any concrete conclusions about this. It's just something to ponder...