The Muslim woman is allowed to take pills to stop menses during Ramadan. If she does so to continue observing fasting, then her intention is a good one and Allah will reward her for such a good intention.
In an attempt to furnish you with an answer to your question, we would like to cite for you the following fatwa issued by the late Sheikh Sayyed Mutawalli Ad-Darsh, former head of the UK Shari`ah council, in which he states the following:

The majority of Muslim scholars, as far as I know, do not consider such an act as being tantamount to tampering with nature. Rather, it is considered an act of regulation with the aim of allowing women who’d rather avoid their periods in order to participate with people in fasting Ramadan and performing Hajj at ease. There is nothing wrong with women using pills or available resources to block menstruations and have chance to perform their acts of worship during such times.

Moreover, Dr. Rif`at Fawzi, former professor of Shari`ah at Cairo University, adds:

There is nothing wrong, as far as Islam is concerned, if a woman takes such pills during Ramadan in order to be able to continue fasting the whole blessed month without interruption.
however, we have to keep in mind that a woman is allowed to do so while being guided by the following conditions:
1. Taking such pills must not result in putting her life and health at risk.
2. It is better, before taking such medication, to seek the advice of a reliable physician.