The Crown case is that Preston killed Ms Fan to get sole custody of the couple's two children.
Mr Bullman said when he was in hospital in early 2013, Preston visited him, but always ended up talking about his own problems.
"The conversation would invariably end up discussing Mike's problems, the difficulties he was having, health issues...it was difficult at the time," Mr Bullman said.
"Mike was convinced that Mei was in the country illegally, that she had at least two different surnames and possibly two different dates of birth," he added.
"He furnished that information to Immigration with a view to getting her deported out of the country."
Prosecutor Grant Burston asked Mr Bullman to characterise Preston's mindset about these issues with Ms Fan.
"In a word - obsessive - it was all-consuming for him..."
Mr Bullman said at times when he visited Preston, he'd see "papers scattered all over the table" as Preston struggled with a multitude of official agencies.
Preston, he said, was battling housing authorities as well as Winz, Immigration, Police, his childrens' school and on one occasion, Women's Refuge.
"He didn't seem to pick his battles. He was fighting everyone, trying to get someone to listen to him," Mr Bullman said.
"He invariably lost and it was terribly frustrating for him..."
Mr Bullman said his friend often spoke of profound personal struggles.
"There were a couple of times he said he couldn't go on with it anymore. He was really struggling. Basically he felt no-one was listening to him."
He said Preston "couldn't see there being an end to it all" and was frequently tearful when discussing his problems.
Mr Bullman said Preston mentioned a Scandinavian called "Sven" who he said was seeing Ms Fan.
Preston allegedly said 'Sven' had been to New Zealand and bought the children fishing rods and furnished Mei's house.
"Mike's understanding was that Sven would be coming back at some stage...Mike though he couldn't compete with Sven."
The court previously heard Ms Fan had a Finnish boyfriend, Tani Hoyhtya.
"Mr Preston was fatalistic about Ms Fan having other boyfriends, wasn't he?" defence lawyer Louise Sziranyi asked.
"Correct," Mr Bullman replied.
After Mr Bullman gave evidence, Detective Constable Jana Peterson, who investigated Ms Fan's social media and other online accounts, told the court he found no evidence of arguments with people on Trade Me about bad trades.
Earlier, Salvation Army Captain Malcolm Robinson said he first met Preston around 2012, and he came to church services once a fortnight or once a month.
Mr Robinson said Preston was often emotional.
"He would get quite emotional but then he'd sort of level off as the day went on."
But on the weekend Ms Fan's body was found, Preston's mood "was more extreme" and he was "stressed to the max," the court heard from Mr Robinson.
Detective Kyle Sherson said on November 10, 2013, he was alerted to a possible homicide at Ms Fan's Miramar home.
Later that day, he was tasked with arresting Preston for breach of a protection order.
Mr Sherson said he did not tell Preston about the death of Ms Fan.
"Oh God, they were only text messages," Preston allegedly told the detective when he was arrested.
Mr Sherson said Preston quickly grew "flustered and panicky" and asked to use the toilet otherwise he would "piss himself."
The detective said he considered this an unusual reaction from someone "who'd just been arrested in this manner."
After arriving at a police station, Mr Sherson told Preston that Ms Fan had been found dead.
"Mr Preston began to wail and cry and this continued for several minutes," the policeman told the court.
But Mr Sherson said Preston then composed himself.
A recording of a tearful interview with Preston at Wellington Central police station was then played to Justice Joseph Williams and the jury.
The day Ms Fan was found dead, Mr Sherson told Preston the interview related to the alleged breach of a protection order against Ms Fan.
"Actually, I'm confused. Did you say protection order?" Preston asked. "Don't you mean trespass order? The protection order was only handed to me a couple of days ago."
Preston told Mr Sherson police "scared the sh*t" out of him and his children when the officers arrived at his house.
In the interview, police told Preston they'd found his "ex-partner" Ms Fan dead at her house.
Preston asked "how" and wasn't told, but said: "I warned you guys about that."
He began breaking down in the interview room, and told police Ms Fan "constantly said she was going to kill herself."
In the High Court, Preston appeared on the brink of breaking down again as the interview was played to the court.
"You guys said everything was okay," Preston told the police interviewing him in 2013.
He cried and bawled in the interview room, telling police Ms Fan was "just a pussycat" and not the strong, tough independent woman she thought herself to be.
"We split up. We split up," Preston told police, before crying loudly again. "Oh God, it's just not right. It's just not right. It's just not right."
Preston said the pair split up on November 22, 2010, the day he left China with their two children.
"I felt sorry for her. That's why I brought her to New Zealand."
Preston told police Ms Fan had no friends in New Zealand, and her uncle "had a contract out on her."
It appeared this was due to a dispute over 500,000 Renminbi (about NZD120,000), Preston told police.
"I hate him. He's an ars*hole. Such an ars*hole."
All my money went into her bank account," Preston said in the interview.
He then catalogued a list of family tragedies, including the deaths of relatives.
Preston said Ms Fan arrived in New Zealand on September 28, 2011, because he and the children were already in the country.
He said Ms Fan then moved out of his Ahuriri St home on October 10, 2012.
Some twenty minutes into the interview, Preston again started sobbing.
"She never admits that she's not happy...that she feels alone...I try and comfort her and go around and see her, and then we end up in an argument."
He said Ms Fan was concerned with not "losing face" and so hated to show weakness.
"There's nothing left after losing face..."
We were just there for each other. She was sad, I was sad. We used to have make-up sex, you know?" Preston added.
The policeman signalled he understood.
"She went all crazy, and started making death threats to me. So I reported her."
Preston said Ms Fan did not react well, calling him names.
"She just went septic on me, all nasty. I asked her does Tani know about us? Tani's her boyfriend."
He referred to United Nations employee Tani Hoyhtya.
Preston then told police about what sounded like a Ponzi scheme, in which people lost money.
He again couldn't finish a sentence, and appeared to vomit in his mouth.
THE TRIAL OF MICHAEL PRESTON:
October 19 : On the trial's first day, Crown prosecutor Emma Light tells the court Preston killed his wife in a "frenzied knife attack" to get sole custody of their two children. The prosecutor claims the couple had a "fractious" relationship peppered with violent outbursts.
October 22: The court hears Ms Fan once claimed Preston dragged her by the ankles to a balcony five floors off the ground, and held her over the edge.
October 23: Immigration NZ analyst Aleli Go tells the court Preston asked for Ms Fan to be deported in 2013. , Mei Fan once dragged her by the ankles to a balcony five floors off the ground, where he then held her over the edge,
October 28: Detective Graeme Muir tells the Court Preston discussed custody issues and the death of Ms Fan on his Facebook page. "Bad enough losing a wife but also having my kids taken from me at the same time," Preston wrote on December 21, 2013.
