Why did the police officer come to your house and ask you to take a friend of your son down to the station? This does not make sense! If the friend is an adult the police should have contacted him direct; if he is a child they should have contacted his parents - not you! If what you have said is a correct statement of the position, then you should refuse on the grounds that you are not responsible for your son's friends.
You should also contact the friend's parents and let them know what is going on - including giving them the information you have from the witnesses who say it was someone else who made the remark.