In OOtP it tells of a time where Sirius discovers that Snape is following them at night to find out where they go(the Marauders). Sirius tells Snape how to calm the whomping willow to get in the shrieking shack knowing that Lupin is in the form of a werewolf and will kill him. James finds out Sirius had done this and stops Snape just before he goes in. They imply that this incident brings them before Dumbledore because now Snape knows that Lupin is a werewolf. Anyway I believe Snape told Lily thinking she would be repulsed but instead she sees that James wasn’t as bad as she thought. JMO
Here’s a link to an interview with Daniel Radcliffe and his opinion of DH. It’s pretty good.
http://www.ew.com/ew/article/0,,20044270_20044274_20048635,00.html