Sorry. Even if we could help you with that, we won't discuss methods to bypass security.
While I applaud your desire to help the person who lost their phone, the truth is that unless they put their phone number on the lock screen, there's not much you can do to unlock it.
That said, you could do the following: Keep it charged so that if they try to contact it using Android Device Manager, it's online. You could also leav it in a visible area so if it rings, you might see some of the caller's information and then you can call them (if it shows a phone number in the notification). Other than that, there's not much you can do. Carriers and local law enforcement are just not in the business of reuniting people with lost items.