How do you expose a coworker who pretends to know their job but copies from you?
When it comes to lackadaisical employees you must not try to catch them in a web of deceit. Try to view it from the manager's perspective. Management only wishes to keep on staff those who are contributors to the company. If someone is a fraud (and a copycat) you should direct the problem and find the solution. You can do this in four easy steps.
1. Gather Evidence: Please always be sure to gather some form of evidence indicating this mishap. This releases you from it becoming "gossip" or on grounds for easy dismissal from upper management. It also shows the other two parties that you are serious about it being taken care of. People who like to complain never want to do the necessary work it takes to gather evidence, this shows your boss and your coworkers that you are serious.
2. Confront the person: It is highly important to first talk to the coworker and see their side of the problem. It is highly unlikely that they will confess and ask forgiveness, thereby changing their ways and becoming a valuable asset to the company again. However, it is worth a shot. If someone hits a slump they deserve a chance to get out of it again.
3. Confront a direct superior (manager): Your management needs to know what is going on. If someone is riding coat tails in the company they must be confronted by a boss. The most important thing for that manager to do is begin a paper trail. This instance needs to be documented to show the problematic employee how serious the company is about integrity and also to be the start of the evidence file should the problem be severe enough to necessitate dismissal.
4. Follow up: This is particularly the most important part. You need to check back with the supervisor and the fellow employee as time goes on. It is important to have a stable relationship with your coworker to ensure a healthy work place. It is just as important to follow up with the supervisor to ensure that this is still not happening and that the proper steps were taken to take care of the situation.
By following these four steps you can ensure you will not be drawn into a "he said, she said" quabble which I've seen last for years. More importantly it will reflect well on you as a worker. Managers love to see employees stepping up and going above and beyond to ensure the interests of the company and the employees are maintained.