If there is a possible threat to you physically or monetarily then you are allowed to listen and even inquire with others if what you heard is true in order to take proper precautions for your safety. Even if it is just a feeling you have that something is wrong, like someone looked at you the wrong way or you have any inkling he hates you and want to harm you, you have a right to investigate and even inquire to see if you or your property are in danger and take proper precautions. This is not considered speaking rechilus even if it sounds like you are making someone look bad, but you have every right to protect yourself. But that is all you are allowed to do. You can’t accept anything you hear as truth and act upon it even if you hear the same thing from many people. He should be treated as any other Jew you are just allowed to protect yourself and property from harm’s way. For example, you can’t hate him in your heart and if he asks for a loan or tzedaka you must give it to him. If you don’t then you will be transgressing don’t take revenge or bear grudge especially since he has not done anything to you yet. You also can’t embarrass or treat him any less than any other Jew besides taking precautions to be sure you don’t get hurt. Even if there are rumors that he told on you to the government and they might now be after you, which in that case he is not considered part of “your nation” and you can withhold things from him bit until you know for sure he did it you have to treat him as any other Jew and just protect yourself. The Chofetz Chaim bring a gemara on Shabbos 56a as proof that you can listen and inquire of people to protect yourself from the story of the prophet Shmuel telling King David that he is allowed to listen to and cross check Tzova who was a known enemy of Mefiboshes about whether Mefiboshes, a relative of Shaul, was rebelling against the king and deserves to be caught and sentenced to death. King David only had a slight indication of a rebellion from the fact that Mefibishes didn’t attend a feast King David invited him to, but that was enough to be allowed to inquire about his motives and the potential threat. Those this might have been a more serious matter, a threat to life, but the Chofetz Chaim felt this was a good enough example to prove that any threat physically or monetarily can be researched, inquired into with taking proper precautions as long as you don’t accept it as truth until knowing for sure on your own, with accepting it from other that there is a problem.