Must a woman whom Allah has guided after she had been a polytheist—and she used to fast Ramadan and abstain from fasting during her menstrual period without making up those days—make up the days she did not fast during her menstruation before her guidance?

Allah does not accept deeds from a polytheist, as faith is a prerequisite for the acceptance of deeds, as stated by the Almighty: ﴿And it was already revealed to you and to those before you that if you should associate [anything] with Allah, your work would surely become worthless, and you would surely be among the losers.﴾

Therefore, if what is meant is major shirk (definition), the woman is not required to make up the days. But if what is meant is minor shirk, then she is required to make them up because it does not prevent the acceptance of deeds.