A fun addition would be an anonymous karma system. Tips that the boss finds helpful will be upvoted, tips that seem petty or false can be downvoted, and future tips will appear with credibility ratings depending on the anonymous tipster's karma, but without revealing their name.

Also, indicate in a vague sense how many prior complaints the tipster has submitted without giving an exact number. This helps with the concerns about workplace paranoia, since you don't have to worry about running afoul of the few idiots that make up lies about everyone they don't like.