Heather Duke: Martha Dunnstock and Heather Duke were close friends during their childhood. It isn’t explained why the friendship ceased, but it is made clear that Heather Duke does not want anyone at Westerburg High to know or remember their past friendship. Duke goes as far as mocking Martha for attempting suicide, showing no remorse or sympathy.
Veronica Sawyer:
Veronica and Martha’s first interaction is when the former is forced by Heather Chandler to forge a love note from Kurt to her, which leads to Martha being ridiculed when she walks up to him and shows him the letter, in which he laughs at her, causing her to storm out of the cafeteria. Despite making the note, Veronica did not laugh at the ridicule and becomes fed up at being cruel to people, especially Martha. At some later point, Veronica calls her fat (As the Heathers were doing the same, and she was being influenced), but tries to apologize afterwards. Martha cries and believes her apology to not be genuine.
Veronica, hearing of her suicide attempt, is very worried and asks Duke if she had survived. Duke responds and confirms she survived, but then mocks her for “trying to be popular”, to which Veronica, in defense of the absent and injured Martha, slaps her.
At the end of the film, free at last of the Heathers’ toxic influence, Veronica Sawyer makes amends with Martha and invites her to hang out on prom night and watch movies with her, which she accepts. This is the start of their friendship. https://heathers.fandom.com/wiki/Martha_Dunnstock