Just to clarify Mycat's answer:
As has been stated, a criminal record is for life. Most convictions, however, become 'spent' after a fixed period of time. (See Mycat's list. Note, though, that if the person is convicted of another offence before the first one becomes spent, the first conviction won't be 'spent' until the second one also becomes so).
Once a conviction is 'spent', you don't have to declare it when applying for most types of employment. (If the application form asks whether you have any criminal convictions, you can legally answer 'No' without fear of prosecution for lying on the form). Neither do you have to declare it when applying for things like insurance. Similarly, if an employer requires a Criminal Records Bureau check (at 'standard' level) the conviction will not be listed.
Note, though, that if potential employers or insurance companies happen to know about a conviction anyway, they're not obliged to disregard this information when considering an application for employment or insurance cover.
Some jobs are exempt from the provisions relating to 'spent' offences. These are mainly jobs working with, or alongside, children or other vulnerable people. For these types of employment, the employer can insist that you declare all convictions (and you could be prosecuted if you answered 'No' to a question asking if you have any convictions). Also, the potential employer has the right to insist upon an 'enhanced' level CRB check, which would show the conviction.
Lastly, it's worth pointing out that the provisions of the Rehabilitation of Offenders Act don't apply to foreign governments. If, for example, your friend wants to travel to the USA, he must obtain a visa because the US authorities will never regard his conviction as 'spent', so he'll never be eligible for the visa waiver program.
Chris