I believe that the Bible allows for divorce in some cases. This particular case met two of those circumstances (adultery and an unbeliever unwilling to stay). She is no longer bound to him like the verse says. I do believe that she should remain unmarried. This is the only instance that I can recall in the Bible that speaks to the bond being broken. I know of none that speaks to it still being there when they are divorced through a civil court.
Then you would agree that what God has joined together, man CAN put asunder?