According to the US embassy website, anyone who has ever been arrested (even if totally innocent) or convicted of any offence (except very minor motoring offences, such as speeding) must apply for a visa, irrespective of the type of passport held.
So, if your friend 'does things by the book' he must apply for a visa and, because the Rehabilitation of Offenders Act doesn't apply, he must declare his conviction. (As far as the US authorities are concerned, no offence is ever 'spent').
The fact that your friend's conviction was so long ago might well mean that he'd be granted a visa but it's impossible to be certain. The procedure can be very long-winded. (Someone posted on AB to say that their partner had waited 6 months before being turned down). It's also costly and time-consuming. (There's a non-returnable application fee plus another fee for the courier to return the applicant's passport. Applicants have to travel to London for an interview). The procedure can also lead to inconvenience as the applicant's passport might be held by the US authorities for several months, effectively barring the applicant from leaving the country..
In practice, many people simply 'forget' about their convictions and fill in the visa waiver form on the plane, stating that they've never fallen foul of the law. The US authorities have no direct access to UK criminal records, so they won't know that he's lying.
Chris