9 Yr old boy with behavior issues

I have a 9 yr old boy who has behavior issues. He is on the aggressive side but very calm and follows rules at school. Never had a complaint from school. Does pretty good in school. The issue is at home, with friends, and social functions. Recently, I met up with a friend. I left him with my friend for less than 5 minutes since I had to go check on my 6yr old daughter. I came back and was shocked to find out the things he told my friend. He told my friend that he and his friend have a secret diary and write things against her. My son further told her that he and his friend's dad call her b***. I am in shock and asked him why he said such things and have never told me. This was surprise for me.

He confessed that there maybe one thing that he made up and was joking. He told me later that there is no diary and he made that up too. I really don't know what to do. I have given him timeout and am really watching him. My friend of 7 years is no longer talking to me since no one likes to be cursed at by kids.

I really want to find out what I can do to fix his behavior issue. There are other incidences in the past but nothing as big as this one. Ahh.....