What is the ruling on someone who prayed alone, then their intention inadvertently changed to that of a follower, and what is the ruling on their prayer when they followed an Imam who pronounced the letter Dhād (ض) as Dhā’ (ظ), knowing that they quickly ceased following?

The question is unclear, and we will assume that the worshiper began the prayer individually then intended to follow an Imam:

Validity of the Prayer: Scholars differed regarding one who began the prayer individually and then intended to follow an Imam. Some said his prayer becomes invalid, while others said it does not. Ibn Uthaymeen preferred the view that it does not become invalid. Thus, the prayer of one who transitioned from praying individually to praying behind an Imam is valid. Breaking the Followership: Returning to praying individually after intending to follow an Imam does not harm. Many scholars permit breaking the followership and leaving the Imam, even without an excuse. Praying Behind One Who Pronounces Ḍād as Dhāʾ: Praying behind one who pronounces ḍād (ض) as dhāʾ (ظ) is valid according to many scholars due to the close proximity of the two letters' articulation points and the difficulty of avoiding it. One should pray behind him to gain the virtue of congregational prayer while gently advising him.