The man should start the process of Li'an.
حَدَّثَنِي مُحَمَّدُ بْنُ بَشَّارٍ، حَدَّثَنَا ابْنُ أَبِي عَدِيٍّ، عَنْ هِشَامِ بْنِ حَسَّانَ، حَدَّثَنَا عِكْرِمَةُ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ أَنَّ هِلاَلَ بْنَ أُمَيَّةَ، قَذَفَ امْرَأَتَهُ، فَجَاءَ فَشَهِدَ وَالنَّبِيُّ صلى الله عليه وسلم يَقُولُ " إِنَّ اللَّهَ يَعْلَمُ أَنَّ أَحَدَكُمَا كَاذِبٌ، فَهَلْ مِنْكُمَا تَائِبٌ ". ثُمَّ قَامَتْ فَشَهِدَتْ.
Narrated Ibn `Abbas: Hilal bin Umaiyya accused his wife of illegal sexual intercourse and came to the Prophet (ﷺ) to bear witness (against her), (taking the oath of Lian). The Prophet (ﷺ) was saying, "Allah knows that either of you is a liar. Will anyone of you repent (to Allah)?" Then the lady got up and gave her witness.