If the wife has lived in her new county for at least three months, absent an agreement to the contrary, the case will likely be heard in her new county of residence. If she has not yet lived in the new county for three months, then she would not have established residency in that county for filing a separation case and her former county of residence (the husband's county) would be the proper venue.
If there are additional complicating factors, such as a child custody case, then the wife's new county may be receptive to the argument that it should transfer the case to the husband's county, based upon that being a more convenient forum for deciding issues of custody based upon the presence of certain witnesses or evidence in that case. That's something to potentially discuss with a divorce lawyer.

