After performing ablution recite the following 3-times.
بِسْمِ اللَّهِ الرَّحْمَٰنِ الرَّحِیم
لََا اِلٰہَ اِلََّا اَنْتَ سُبْحَانَکَ اِنِی کُنْتُ مِنَ الْظٰلِمِیْنَ
اَٰللّٰه نُوْرُ الْسَمٰوٰتِ وَالََْأرْضِ
لََا تَاخُذُہْ سِنَۃٌ وَّلََْنَوْمٌ
Blow on a cup of water and let that person drink this water who is having an illicit relationship. This is required to be done for 40 days.