The USA doesn't recognise the UK's Rehabilitation of Offenders Act, and has no corresponding legislation of its own, so there are no fixed time periods for offences to become 'spent' under US law. (Indeed, US law doesn't recognise the concept of 'spent' offences at all. Offenders are stuck with them for life).
"Assault with intent to commit serious bodily harm" (which is the wording used in the USA) is classed as 'moral turpitude' by the US authorities. Under the USA's Immigration and Nationality Act, anyone who has committed an offence involving 'moral turpitude' is automatically banned for life from entering the USA but it's possible to apply for a 'waiver of inadmissibility'.
So, in the first instance, your friend would need to apply for a US visa (providing an official copy of his police record and attending an interview at the US Embassy in London). That procedure alone can be lengthy and expensive (particularly if he has to take time off work to attend the interview and then travel from the other side of the country). The US Embassy staff will then automatically refuse to issue him with a visa. (They have no discretion. Their Immigration and Nationality Act forces them to do so). They will then offer to refer the matter to Washington though, as an application for a 'waiver of inadmissibility'.
The processing of an application for a waiver is extremely lengthy, taking at least six months and usually over a year. One AB member reported that her husband had to wait 15 months before having his waiver application refused (and he only had two convictions for driving without insurance).
As I stated at the start of my post, there are no fixed time periods before offences can be 'ignored', with the US authorities responding to questions by just (unhelpfully) saying that every application is considered individually upon its own merits.
However, based upon what I've seen written on AB and elsewhere, I doubt that there'd be any point in your friend trying to enter the USA until at least 10 years after his release from prison. Given that he might have to wait a month or two to get an interview at the US Embassy, and then wait possibly 15 months or more for his waiver application to be considered by the authorities in Washington (and, obviously, that he wouldn't want to book any travel until he knew if he'd got a visa) he'd need to commence the visa application process around two years before his planned date of travel.
Even after a ten year wait though (followed by two years for the processing of his visa application) it's probably still unlikely that he'd be allowed to enter the USA.