Natalia Darkowska, 18, was found guilty of perverting the course of justice after university dropout Jesse McDonald, 25, killed Naomi Hersi in March.
Their Old Bailey trial had heard how former A grade student and tennis ace McDonald contacted 36-year-old Ms Hersi through dating website Fabswingers.com.
After days together, McDonald drugged and stabbed the hospitality worker to death with a knife and a broken bottle at a hotel near Heathrow Airport where he was living.
The intelligent and well-spoken defendant then spun a web of lies to his mother and called on his girlfriend to help.
On Friday March 16, McDonald had texted his girlfriend saying that he had been an “idiot” and that he had “not cheated” but had “got addicted to a nasty drug” that he claimed he had never had before.
He continued to message Darkowska, saying he had “got a big drug dealer lying across my floor right now” who “won’t wake up”.
Claiming not to have slept in three or four days and that he had a knife held to his throat by Ms Hersi, McDonald gave the impression that he was in danger.
Once she arrived, Darkowska set about cleaning up the blood as Ms Hersi’s body lay in the bathroom.
The defendants left around eight hours later and took a taxi to a hotel in Teddington, with McDonald fearing police were closing in.
Police found the victim lying half-naked, partly covered by a rug on the small bathroom floor and later tracked down the defendants.
At an earlier hearing, McDonald was jailed for life with a minimum term of 20 years for murder and perverting the course of justice.
In mitigation for Darkowska, Jemma Levinson said she had been a “model student” who was “studious and respectful” and had been “misled and lied to” by McDonald.
The court heard the offence was “totally out of character” and she was “besotted” with McDonald.
The lawyer said: “She still does believe the account she was given by Mr McDonald. This is clearly the case where whatever Ms Darkowska did she did in the belief and understanding that she was helping someone she loved but somebody she believed was the victim of the most serious offences.”
She added that she believed McDonald had been through a “terrible ordeal” and was an “innocent man”.
Ms Levinson said Darkowska had been “under the spell” of her boyfriend, who had been taken in by her family as a “trusted member”.
As a result of the trial, she had a “breakdown”, the court was told.
Darkowska, from Trowbridge in Wiltshire, was handed a suspended jail sentence of two years in what were described as exceptional circumstances.
She was also ordered to do 150 hours of unpaid work.
Judge Mark Dennis QC said McDonald was a “dominant” character and Darkowska had become “entranced by him”.
The judge said McDonald was “deeply flawed” and capable of misleading.
While she may have set off to lend emotional support, the judge rejected the suggestion she never saw the body.
He told her: “One of your references refers to this being a once-in-a-lifetime mistake. Make sure it is.”
Detective Inspector Tom Dahri, from Scotland Yard, said: “Although Darkowska did not inflict any harm on the victim, she is just as complicit in the attempted cover-up and trying to help McDonald escape.
“When McDonald called her to help cover up his actions, Darkowska came straight to London from her home in Wiltshire and attempted to clean the bloodstained walls and carpet.
“She didn’t call police or attempt to get the victim any help - she only thought about her boyfriend.”