In Wisconsin is it legal to call someones work and say that you think that they are using drugs?

I'm pretty sure that a person I know is using drugs. They go to work drunk from the night before and all of that! This is my childs father and hes trying to get custody of my son. I dont think that he is a very safe person to be around right now until he gets some help!!!! This would also be evidence that he is using drugs. He swears up and down that he's not. I know he's lieing. No one will do anything about it because they have no reason to believe that he using. I'm worried he might get some custody and put my son in danger. He has had drug charges and drunk drivings from a couple years ago though. What if he stopped using for a little bit because he knows court is coming up for this. Then they test him and he's clean??????