David Sole, 33, of no fixed address, had denied murdering Jonathan Ellison but was found guilty at Lewes Crown Court of beating him to death.
Denise Antonia, 33, of Appledore Road, Brighton, was cleared of murder but found guilty of assisting an offender. She was jailed for five years.
Mr Ellison was found dead in his flat in Gloucester Street on 8 April 2011.
Sole and Antonia were arrested in Winchester, Hampshire, on 16 April following an investigation by Sussex Police's major crime branch.
Following the two-week trial, Sole was told he would serve a minimum term of 18 years.
Det Ch Insp Trevor Bowles said: "Jon Ellison was killed by a man who had befriended him and who he trusted.
"His trust led to his death, as the result of a motiveless but brutal drug and drink-fuelled assault, which caused some of the worst injuries any of us have ever seen."
Mr Bowles said Mr Ellison was a "vulnerable man with many health and personal problems" but he was regarded as "a kind and trusted person".
In a statement issued through Sussex Police, Mr Ellison's brothers said: "He did not deserve to be taken in such a brutal way.
"It was completely at odds with his kind and gentle spirit."

Imani Wiltshire was discovered on the lower deck of the bus in Stratford, east London, on 28 September.
She was taken to hospital where she was pronounced dead. A post-mortem examination found she died from head injuries.
Rosalin Baker, 25, and Jeffrey Wiltshire, 52, of Manor Park, pleaded not guilty to murder at the Old Bailey.
They also denied charges of causing or allowing the death of their child.
A trial date has been set for 13 March at the same court.

However, the Bairns boss has underlined that any forward signing will need to exhibit even more quality than two of his promising youngsters.
"If I bring another striker in he's got to be better than young Botti Bia-Bi and Scott Shepherd," said Houston.
"I would be looking for the more experienced type, and another defender would come in handy as well."
Eighteen-year-old Bia-Bi, a London-born Scot who has progressed through Falkirk's academy, glanced in a fine equalising header against Cowdenbeath on Saturday to ensure Houston's side left Central Park with a point.
The former Dundee United manager stated in no uncertain terms that he wants to "eradicate" the errors that allowed Cowdenbeath to lead twice in their first Championship game of the season.
Houston labelled "not picking up runners" as the problem for conceding the opening goal and a "crazy mix-up" as the reason former Bairn Sean Higgins gave the Blue Brazil a second.
An "exceptional" Rory Loy strike and Bia-Bi's header kept Falkirk on level terms.
"To score a header like that is not the easiest thing," added Houston. "He put it right in the corner so credit to him.
"He made an impact when he came on and he might be due a longer run from the start.
Next up for the Bairns is the visit of Rangers on Friday.
A man has been jailed for life for the "motiveless and brutal" murder of a Brighton man in his city centre flat.
[NEXT_CONCEPT]
The parents of a three-month-old baby found seriously hurt on a bus have denied murder.
[NEXT_CONCEPT]
Peter Houston is still seeking to fine-tune his Falkirk squad, with a striker and defender pinpointed as priorities.