الجواب وبالله التوفيق
As per the situation described in your question when the period starts during the usual days (of her habit) then she should consider the first ten days from the start as haidh and refrain from salah etc. during these days. After the completion of the ten days any blood that continues will be considered as istihadah(i.e. spotting, dysfunctional uterine/vaginal bleeding). Until a fix habit does not develop the above method shall be used.
واللہ اعلم بالصواب