What is the ruling of Islamic law regarding the dissolution of the marriage contract and the paid dowry, after the fiancé's adultery and corruption have been proven, and is it permissible to monitor him for verification?

Accusing a Muslim of adultery without clear proof is not permissible, and spying is forbidden except to prevent a major evil or avert a great corruption. If it is established that the man is a dissolute person well-known for adultery, then the wife or guardians may refer the matter to the judge and request the annulment of the marriage. If the judge annuls the contract before consummation, the wife is not entitled to any part of the Mahr (dowry), and the entire Mahr returns to the husband.