Yes, it is ethical to spy on a person, if they are committing a crime. If a person is breaking the law or is violating probation or a court order, there is nothing wrong with simply watching the person when they are out in public and going about their business. This makes sure there is law and order in our society.
Regardless of justifications ranging from personal or financial protection to national security it is never ethical to spy without solid supporting evidence and a warrant. The right to privacy is one of our sacred rights of living in a free country. Obviously there are warranted situations involved with police work that supersede our individual rights but we must preserve our rights if we are to remain a free society.