The N-Dubz couple are believed to have separated after repeatedly clashing on a trip to the Maldives earlier this month.
Fazer and the Topshop heiress - who clashed with Contostavlos last year when the X Factor star got her removed from a section of a nightclub - have been seen partying together in London on two separate occasions this week, prompting suggestions that she could have become entangled in their split.
"Tulisa and Chloe had their differences after a misunderstanding which left Chloe absolutely seething," a source told The Mirror. "There were claims Tulisa was 'rude' but she genuinely didn't know who Chloe and her pals were when they were cleared from her table.
"Tulisa would never want to try to dictate to Fazer who he can and can't hang around with, but she's seething that he keeps being spotted with that woman. It is rubbing salt into the wound."
The insider added that friends of Contostavlos and Fazer expect them to reunite in the near future, though he may be forced to abandon his friendship with Green as a result.
"Chloe and Fazer are just pals, but it's unfortunate timing for them to be together given the history between Chloe and Tulisa," they said.
"Knowing those two, they'll be back together again in a few days. They often have their ups and downs. But if Fazer wants to win her back, he might want to stop socialising with Chloe."
Green has denied speculation of a romance between herself and Fazer, writing on Twitter: "I just don't get why every guy who is my friend is suddenly someone I am supposedly with.. Girls can have friends that are guys! #notacrime."
Contostavlos admitted in a recent interview that she gets "jealous" of Fazer's female friends.
She said: "If he's seeing his girl mates, I want to know them. I say, 'What friend? You don't need female friends'... I don't stand for any s**te."
Watch the N-Dubz 'I Need You' video below: