As others have said, legally there is not much your friend can do if you were to take this idea and run with it. Ideas are not worth the paper they are written on, as they say.
However, it might be best to approach your friend and discuss your proposals with him. I wouldn't let on that you think the idea is golden, but rather downplay it slightly, but say you think the idea might be "viable" and are considering making the site mentioned.
The chances are your friend will be happy for you to use the idea, on the other hand he may choose to assist you with the site. Depending on his level of involvement you can then negotiate exactly how much of a share to give him.
This way, you may get the business and keep your friend. If negotiations don't work out satisfactorily for you, then go off and do it on your own anyway!