Praise be to Allah, and may Allah’s peace and blessings be upon the Messenger of Allah, his family and his companions.
As to what follows:
In response to your question, we say, and with Allah Almighty lies all success:
If she left praying based on the continuation of menstruation, then it later became clear to her that it had ended earlier, then she is not obliged to make up the missed prayers. That is because she left off praying due to her regular period of days in which she is not required to pray.
It has been reported in an authentic hadith that a woman came to the prophet (peace be upon him), and she had left off praying for a long period of time. And in some narrations, the period had reached seven years and she was menstruating. So the prophet (peace be upon him) did not order her to make up for her past prayers. That is because she left off praying due to the understanding of not being obliged to pray, and that those days did not require her to prayer, and she was therefore, according to this inference, not from amongst those accountable for prayer.
But if a woman wants to make up her missed prayers to be on the safe side, then that is good. As for it being an obligation on her to make up what she missed, then she is not obliged to make up for what she missed during what she assumed was a state of menstruation, or nifas (post-partum bleeding), which she thought had not ended, then afterwards she realized it was spoiled blood (not related to menstruation), or that she had become pure without realizing, etc.
And Allah knows best.