What is the ruling on a wife fasting Ramadan based on a doctor's fatwa that what she was experiencing was irregular bleeding (istihada), but it was later found to be menstruation?

The basic ruling concerning blood that returns after years of cessation is that it is menstrual blood, as long as its duration does not exceed fifteen days. The wife must make up the days she fasted, because the fast of a menstruating woman is invalid by consensus.

In matters of Sharia, one must refer to scholars, not doctors.