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.
The husband has the right to take back his first wife, and his second wife has no right to prevent him from doing so. The fact that the second wife threatens her husband to establish a sinful relationship with another man; constitutes a blatant act of rebellion and disobedience. The Sharee’ah clarified how to deal with a disobedient wife, so you may refer to Fatwa 132053.
Indeed, the steps that the Sharee’ah advised to follow in dealing with a disobedient wife are wisdom itself in dealing with such a case.
However, it should be noted that it is not permissible for the husband to beat his wife except according to certain conditions as clarified in Fatwa 85402.
Allaah Knows best.