treat them as a subordinate but the same way you would any other employee. if you are fair and upfront with this person always, they will have nothing to go to the boss about. i know this will be hard to do but it really is the best way. also, if you have to give this person duties, or an explanation of something, make sure another person is present, if you can, so that they can confirm your professionalism in dealing with this person. keeping a written record is also a very smart way of dealing with conflicts or actions, so that if this person says something to your boss, you have a written record of yours and thier actions