What should I do when I know that a Muslim is stealing from his Christian employer without his knowledge?

There is no disagreement among Muslim scholars that stealing the wealth of Dhimmis or those non-Muslims with whom a treaty has been made is forbidden. If a Muslim knows that someone is stealing their wealth, it is obligatory for him to forbid that person from doing so and to recover the stolen goods if he is able. If the stolen goods cannot be recovered except by exposing the thief, then that becomes obligatory.