Check with your state labor department to find out if the company has an overtime liability. The supervisor may not be aware of the legal ramifications (I know my sisiter's company had to pay overtime in VA for an employee who worked unauthorized hours, but this will vary by state. BTW they fired the employee since he wouldn't stop doing this.) as most supervisors can be relatively clueless about labor law.
Several things concern me about this situation beyond the liability. First, what is he doing during those hours? Depending on the position he is in, this is a perfect opportunity to commit fraud or do something else underhanded that you don't want the company to know about. If he is just doing work, is the supervisor aware that his workload is 50 hours a week vice 40? This could make a real difference if he leaves.
As to whether you should speak up and who to speak up to? Well it could be risky to your own career to speak up if the other employees start to consider you a 'narc'. On the other hand, by providing management with information they need about a potential liability, they may be grateful. A lot of whether it is a good idea to speak up depends on your supervisor, the situation and the company culture. If the supervisor doesn't seem like he would be approachable on this issue, you could bring it up the HR and if they have an annonymous hotline, that might be the best place to bring it up.
If you think the supervisor knows he is working but may be clueless about the ramifications, then research them and tell him casually that you ran across this information and isn't it interesting. Then let him decide what to do. If the suprevisor is someone you don't trust, he may be in on a deal of getting the guy to work unapproved overtime and splitting the eventual profits. In this case HR is your only bet.
(You know having worked for an audit agency does some nasty things to your thinking process! These things spring to mind because I have seen them happen in the workplace and seen people go to jail when they got caught.)
You should question why you want to bring it up though? Does this person get special treatment or accolades for all the work he accomplishes when you know he is putting in more hours than everyone else? Why are you concerned about what he is doing? How is it harming you? Are you worried you wil be forced to do overtime if he leaves? Why is it your business?