تَظُنُّ سَلْمَى أَنَّنِي أَبْغِي بِهَا بَدَلًا
Salma thinks I am seeking someone to replace her.
Tazunnu is a present-tense verb and Salma is its subject. The clause annani abghi biha badalan conveys the content of her supposition. Up to this point the attributed belief is Salma's.
أَرَاهَا فِي الضَّلَالِ تَهِيمُ
I see her wandering in error.
The subject of araha is the speaker, with -ha referring to Salma. This clause communicates his view instead. The absence of coordinating wāw helps prevent the two attributions from being confused.