So a bunch of friends are always pitching ideas to each other.
One day, one of my friends does one of these "this would be a great idea for a site". It's blown off as another of his 'ideas', but to me, it sounds like pure gold.
If I just go off and make it, does he have any recourse, aside from being a little jealous/angry that the site is not his, even though he may never have ended up making it?
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!
To enforce intellectual property rights, one must take steps to protect those rights. It is not automatic. In the US, there is little or no legal recognition of moral rights of authorship. That may mean your friend needs to consider filing a patent or copyright application, or using non-disclosure agreements. Absent these, your friend probably has little legal recourse.
Legally you can use his idea.
I suspect you will have two outcomes: