The main character (MC) is reborn into the role of a side character in a game/novel? This side character is the game/novel's female lead's best friend, who dies, and this is the beginning of the novel/game. The novel's female lead (FL) then goes to visit her late-best friend and from there she meets the male leads. When the MC meets the FL, something is off and it seems like the FL may be aware that she is in a game/novel.
This could be The Reason Why Raeliana Ended up at the Duke's Mansion.
The story matches your description pretty well:
The main character is killed in the real world and wakes up as side character in a novel. In the novel, the character she is possessing is killed by her fiance and her death becomes the reason for the female lead (of the novel) to return to the city and subsequently meet the male lead while investigating her death.
The main character disturbs the story by entering a contracted engagement with the male lead, to break free from her fiance.
In the novel, the MC and the FL are friends, but the FL does not really act as her friend, and seems to be aware that the world has been altered.