To apply for naturalisation, he needs to meet the residency requirements (amongst other requirements):
Residential requirements
To demonstrate the residential requirements for naturalisation, you must have:
been resident in the UK for at least five years (this is known as the residential qualifying period); and
been present in the UK five years before the date of your application; and
not spent more than 450 days outside the UK during the five-year period; and
not spent more than 90 days outside the UK in the last 12 months of the five-year period; and
not been in breach of the Immigration Rules at any stage during the five-year period.
With regard to the italicized text, your friend can apply for discretion to waive one of these requirements so long as he meets all other requirements for naturalisation and he can demonstrate that he has established his home, family and substantial part of his estate here (in other words, he needs to establish that he's "thrown his lot" into the UK - e.g. he had a house here in his name (rented/owned), his career/business was here, his family (wife/girlfriend/children) are here, his pension/assets are here). They would also expect him to have been resident in the UK for 7 years prior to the date of application. See
http://www.ukba.homeoffice.gov.uk/sitec ... let_an.pdf
He would set out his request to apply for discretion at question 6.8 continuing onto page 15 if more space is needed to demonstrate the above and explaining why he was outside of the UK.