All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
The spending on the studies of the wife does not fall within the obligatory spending (Nafaqah) on her husband. To know what the obligatory spending on the wife is, please refer to fatwa 85541.
If the husband volunteered out of his kindness and helped his wife in her studies, then this is good, and this strengthens their marriage and increases affection between them and it would thus benefit them both.
However, in case the husband spends on the studies of his wife without doing so as a donation to her, then he has the right to take back whatever he had given her for her studies.
Allah knows best.