All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
According to the Maaliki and Hanbali scholars the fasting of your wife is valid and she is not required to make it up, since she completed the day without menstruating. The Hanbali scholar ‘Abdur-Rahmaan ibn Qudaamah Al-Maqdisi said, "We mentioned that the period of purification during menstruation is deemed valid. When a woman discharges blood on one day and then she experiences a state of ritual purity in between, she is to combine the days when there is a blood discharge to be regarded as menstruation and the in-between days when there is no blood discharge to be regarded as purification…"
Allaah Knows best.