Yes but you'd have to be sure he really understood he was not to use it; why would he have permission to take it otherwise? Not saying that is not true, just that there would be a presumption that he could use it and you'd have to persuade the police or court that he knew he could not.
No, you could only sue him if it was an illegal weapon. Im not sure if it applies for cell phones but in the 2nd amendment people have the right to have things but they dont have the right to use it, like with guns