The Chofetz Chaim said that if the employer would immediately act, for example fire the guy he just hired if you tell him he is a thief, instead of just being cautious and keeping a good eye on him then you can’t tell the employer, even if the employee might harm him in some way. The Chofetz Chaim brings several proofs that a single person cannot testify or tell the other person if unlawful action will be taken.
1. Krisos 12a: If 2 witnesses say a kohen for example became tamei and he himself says he didn’t he can eat tahor food like teruma in private but not in public and he can only feed himself the teruma but not others because for himself he is believed to say he is tahor but for other and in the eyes of others he ia considered tamei since there are two witnesses against him. Even if others don’t know there are two witnesses he still can’t feed them teruma or eat it with others even though he k knows the truth that he is there since there are witnesses out there that testified he is tamei. Then they won’t accept his testimony that he is tahor. So too, in our cases even if you know this guy is a thief since you can’t do anything in a Jewish court beside making the thief take an oath therefore your words are useless for the employer to take any actions against the would-be thief.
2. Shevuos 30b: If a witness knows that the other witness testifying with him is a thief and is invalid to testify with him, even though if they would testify together the court would accept their testimony because they don’t know that one of the witnesses is a thief, he still can’t testify with him because he knows his friend is invalid to testify and the court would be extracting money based on their testimony unlawfully according to his knowledge since in reality there is only one witness. Certainly, in our case you can’t tell the employer, acting as one witness, if he will take matters into his own hands. Even if what you say is true. Since it would be unlawful to fire the employee based on one witness.
3. Bava Kamma 113b: Rava put a Jew in excommunication for testifying against another Jew as a single witness in a non-Jewish court causing him to pay a non-Jew money. The gemara says if he would have testified with someone else he would not have been excommunicated. It’s clear from this last point that the problem wasn’t that he testified in a non-Jewish court which is only permissible since one of the litigants is not Jewish, and the problem also wasn’t that he testified for a non-Jew to get money from a Jew because then it would make no difference whether he was one witness or with someone else testifying, either way he should have been excommunicated. Rather it must be that he was excommunicated for testifying as a single witness, that if he would have been in a Jewish court, they would not have accepted his testimony therefore he halachachically caused his flow Jew to lose money unlawfully, even if the Jew really did owe the non-Jew the money. So to in our cases, whether it’s a hired maid, a business partner, a shidduch, etc. If you are a single witness and the person you tell will act rashly and not just be concerned do research and be cautious then you can’t tell him lest he takes actions into his own hands and does something uncalled for according to halacha since it would bot have been handled in that way in a Jewish court.