In the supplication of istikhāra, one says:
اللَّهُمَّ إِنْ كُنْتَ تَعْلَمُ أَنَّ هَذَا الأَمْرَ خَيْرٌ لِي فِي دِينِي وَمَعَاشِي وَعَاقِبَةِ أَمْرِي ـ أَوْ قَالَ عَاجِلِ أَمْرِي وَآجِلِهِ ـ فَاقْدُرْهُ لِي، وَإِنْ كُنْتَ تَعْلَمُ أَنَّ هَذَا الأَمْرَ شَرٌّ لِي فِي دِينِي وَمَعَاشِي وَعَاقِبَةِ أَمْرِي ـ أَوْ قَالَ فِي عَاجِلِ أَمْرِي وَآجِلِهِ ـ فَاصْرِفْهُ عَنِّي وَاصْرِفْنِي عَنْهُ، وَاقْدُرْ لِيَ الْخَيْرَ حَيْثُ كَانَ، ثُمَّ رَضِّنِي بِهِ
If You know that this job is good for my religion and my subsistence and in my Hereafter, then You ordain it for me and make it easy for me to get, and then bless me in it. If You know that this job is harmful to me in my religion and subsistence and in the Hereafter, then keep it away from me and let me be away from it, and ordain for me whatever is good for me, and make me satisfied with it.
— Sahih al-Bukhari, Book 19, Hadith 45
Then one should name one's need.
Dreams are not involved in any part of the process. The notion that one will feel at ease with one choice over the other or that one will get a sign or that one will see a dream, etc., is based on a very weak (dā'īf) hadith by Ibn As-Sunni that said one should do seven istikhāras then one would see or feel inclined to one choice over another. Both Ibn Hajar and An-Nawawi commented that that hadith is not to be considered of any value in istikhāra (see Vol. 11, pp. 187 of Fat'h al-Bāri by Ibn Hajar al-Asqālani). Instead, when one performs istikhāra, one should proceed with one's decision and know that Allah will ordain what is best for one as per the hadith of istikhāra by Ibn Mas'ūd (see Al-Futūhat ar-Rabbāniyya 3/357).
If the man and the woman in your question decide to marry, it is not going against the guidance. The istikhāra is about making an action at any specific point in time; it is not about one getting one's perceived or desired results. It cannot change with time as it is about an action, even if the results do not match expectations or the desired results change with time.
The perceived sihr situation of the man and the woman in question is a completely different topic that you should post in another question. When doing so, provide your reasons to believe that magic is involved.