My istant supervisor called me a liar...?

I recently came back to work after a 6 month mission trip. On my first day back, I was unable to log-in to the computer. We commonly use other's log in number to input stats. I asked a friend of mine to log me in while my istant sup. was sitting next to her. He immediately asked me to go get him a cup of coffee (weird request) I dont think he knew we were friends. When I got back into the room, all conversation stopped. The next day, my friend told me that he said I was a liar and not to trust me. She asked me why he would say such things and I honestly have no idea. I am a Christian-I treat people as I want to be treated. I was so shocked and upset, I told my lead supervisor and encouraged her to do the same. She did but now she wants me to drop it. My supervisor said he must take this to HR but my friend said if I do she will no longer be my friend. She is worried of what discrimination she will face by our istant sup. I feel he has been saying these rumors about me to others.