I believe that, because he violated the terms of the Visa Waiver Program he is forever ineligible to use the VWP. They take the attitude that being allowed to enter the US visa free is a privilege, and once you violate the VWP terms you prove that you do not respect immigration law and no longer deserve such a privilege.
The good news is that, as he left before the 180 days was up, he does not have a 3 year ban. He needs to apply for a B-2 visitor visa, which is by no means guaranteed to be granted. To get the visa, among other things, he needs to be able to prove strong ties to the UK (you likely don't count as, being an American, you can just stay there if you chose).
If I were you I would go ask the same question at britishexpats.com, they know a lot about American visas over there.
Good luck!
PS: I'm not a lawyer or anything like it, just a bit of an immigration geek
