Your friend can't legally enter the US because he requires a visa and the minimum time to acquire one is 4 months. (You have to wait, often for several weeks, until you can get an appointment for an interview at the US embassy in London. After the interview, the minimum time to hear whether your application has been granted is 14 weeks).
Your friend will have to risk attempting to enter the US under the VWP (and conveniently 'forgetting' about his criminal conviction). At the present time, the US immigration authorities do not have direct access to the UK criminal records database (although the US government continues to press for such access). Certain information is routinely passed on from the UK to the US but, as the offence is unrelated to potential terrorist activities and does not involve either drugs or sexual misconduct, it's extremely unlikely that US immigration officials will know about it.
Chris