Why? I wouldn't permit my child to "hang out" with a child I knew to be fornicating, and I have already had to prohibit my son from visiting a friend's house when I found out that the friend looked at pornography on the Internet.
I love my children enough to want them to have good role models in their life, not bad ones. What a horrible concept.
My best guy friend in school was a much better Catholic than I was and would never think of having sex outside a committed relationship. The only thing about him that you'd consider a sin is that he sometimes went on a date to the movies with another guy. However, you'd still consider him to be an evil, sinful person.