If menstruation stopped as per the habit, you should start keeping fast after it. And if the blood discharge starts again within ten days it shall be considered that her habit changed. Hence she should stop keeping fast after blood discharge. And if the blood discharges again does not exceed ten days including the habit days then it shall be counted haiz (menses). But, if blood continues after ten days so only the blood discharged as per habit shall be counted as haiz and the extra days shall be counted istihazah (uterine bleeding) and fasting is not forgiven in istihazah, rather she would keep fast in the same state.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband