What is slander?

A former friend let her boyfriend slept at her house while her husband is out of town. Her daughter found out and told me. I knew all about her affair because she introduced me to every one of them. I told her that what she was doing was wrong but she didn't listen. When I found out that she starts bringing her boyfriend to her house while her children are home I got so mad at her and wrote her a letter and called her names. Now, she said she will file charges against me for slander. I'm not the only one who knows about her affair. Her husband doesn't know. Her husband is a great guy and does not deserve to be treated like this. I was friends of both of them but I've lost my respect for her because of this. What would you do if you were in my shoes? and does she have a case to file slander? I have proof of her infedility.