How do you base a story on truth without being sued?

A friend of mine has been arrested on a sexual assault charge under some very fishy circumstances, involving a possible coverup, crooked law enforcement, and a "conspiracy" among his places of employment and the sheriff. I would like to write a work of fiction or non-fiction that tells his story. What guidelines do I need to follow in order to keep the integrity and nuances of the situation without being sued?

[olive]

You can choose to not not use any names. You can do a documentary type work as well where you basically use public records, transcripts etc.