Dr Jessica Mang, 29, is said to have received leaked data from Thomas Ammann, 39, along with Christina Weckwerth, 44, whom he was seeing behind her back.
Prosecutors say the two women made more than £2m in total trading on inside information about Canon's buyout of a Dutch photocopy company Oce.
Ammann was one of just seven bankers at the London offices of Japanese bank Mizuho who knew about the deal before it was announced in November 2009.
Giving evidence at Southwark Crown Court, Mang's flatmate Jean Wang said she raised concerns about the proposed investment in Oce.
She said: "I think when she first mentioned it I was surprised that Thomas had suggested it.
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"I basically said I do not think Thomas should be doing that, that's insider trading."
Ms Wang, who works for Goldman Sachs, added: "The impression I got was that Thomas was using the information he had from working on the private deal and trying to benefit from that."
In a diary entry written after the conversation Mang wrote: "P***** off with Jean. Her 'It's so insider trading' x 2/3. Really? Is it that necessary? Increasingly pissy. The selfish approach".
In the same entry she wrote later "Jealousy? Thoughtlessness? Both? Ah? Shame."
Lawyers for Mang later suggested that she was "clueless" as to what insider dealing meant.
Ms Wang agreed, saying: 'It is not something that she appreciates has significance.'
She first met Mang, who she described as a "really good friend", at school when she was 16 and the pair moved in together in 2007.
Asked about Amman, who Mang met on a night out at the Movida club in Soho, central London, she said: "I personally did not like him very much," she said.
"I personally thought she could do much, much better than him."
She said Ammann had stood up Mang on her birthday and had once left her "stranded" at a Haagen Dazs café for three hours.
She agreed that Mang "did all the running" in the relationship.
Ms Wang said that when Ammann's proposed deal was first brought up Mang was worried about mixing business with pleasure.
She said: "She had brought it up because she was worried that by introducing a money element in to the relationship it would complicate matters."
Prosecutors allege that Ammann used both women for "cash and cover" to make illegal trades.
Ammann, from central London, pleaded guilty to two counts of insider dealing and two counts of encouraging insider dealing before the trial and is not before the court.
Weckwerth, from Germany, denies one count of insider dealing between April and November 2011
Mang also denies one count of insider dealing in November 2011.
The trial continues.