The US immigration service have no direct access to the criminal records of any other country (except Canada, which has a common database with the USA).. EU law prevents the UK from making our criminal records freely available to other governments. Only specific information can be passed on, where there is 'justifiable cause'. If your friend happens to be suspected of terrorism or drug-trafficking the UK authorities will have passed on everything they know about him. Otherwise, the US immigration service won't know about his 'youthful indiscretion'.
Chris