What is the ethical issue involved in peer spying in the workplace?Management wants me to spy.

Management wants me to spy on a colleague. I’ll be using [a spying program] that is 100% hidden, does screen captures, etc. Is there a document out there that I can have management sign to limit my liability? I want signatures from all management stating that they are authorizing me to spy. Thoughts? I have done this before, but this is the first time that they have asked me to compile data against a user for possible use in court. Thanks.

What are some of the questions or concerns you might bring up in an answer and what would you suggest this individual do to respond to them?