Suspicion

Handsome, irresponsible playboy Johnnie Aysgarth (Cary Grant) meets dowdy Lina McLaidlaw (Joan Fontaine) on a train and charms her into eloping despite the strong disapproval of her wealthy father, General McLaidlaw (Sir Cedric Hardwicke). After a lavish honeymoon and returning to an extravagant house, Lina discovers that Johnnie has no job and no income, habitually lives on borrowed money, and was intending to try to sponge off her father. She talks him into getting a job, and he goes to work for his cousin, estate agent Captain Melbeck (Leo G. Carroll).
Gradually, Lina learns that Johnnie has continued to gamble wildly, despite promising to quit, and that to pay a gambling debt, he sold two antique chairs (family heirlooms) that her father had given her as a wedding present. Beaky (Nigel Bruce), Johnnie's good-natured but naive friend, tries to reassure Lina that her husband is a lot of fun and a highly entertaining liar. She repeatedly catches Johnnie in ever more significant lies, discovering that he was fired weeks before for embezzling from his cousin Melbeck, who says he will not prosecute if the money is repaid.
Lina writes a letter to Johnnie that she is leaving him, but then tears it up. After this, Johnnie enters the room and shows her a telegram announcing her father's death. Johnnie is severely disappointed to discover that Lina has inherited no money, only her father's portrait. He convinces Beaky to finance a hugely speculative land development scheme. Lina is afraid this is a confidence trick or worse, and tries to talk Beaky out of it, but he trusts his friend completely. Johnnie overhears and angrily warns his wife to stay out of his affairs, but later he calls the whole thing off. When Beaky leaves for Paris, Johnnie accompanies him partway. Later, news reaches Lina that Beaky died in Paris. Johnnie lies to her and an investigating police inspector, saying that he stayed in London. This and other details lead Lina to suspect he was responsible for Beaky's death.
Lina then begins to fear that her husband is plotting to kill her for her life insurance. He has been questioning her friend Isobel Sedbusk (Auriol Lee), a writer of mystery novels, about untraceable poisons. Johnnie brings Lina a glass of milk before bed, but she is too afraid to drink it. Needing to get away for a while, she says she will stay with her mother for a few days. Johnnie insists on driving her there. He speeds recklessly in a powerful convertible (a 1936 Lagonda LG45[2]) on a dangerous road beside a cliff. Lina's door unexpectedly swings open. Johnnie reaches over, his intent unclear to the terrified woman. When she shrinks from him, he stops the car.
In the subsequent confrontation, it emerges that Johnnie was actually intending to commit suicide after taking Lina to her mother's. Now, however, he has decided that suicide is the coward's way out, and is resolved to face his responsibilities, even to the point of going to jail for the embezzlement. He was in Liverpool at the time of Beaky's death, trying to borrow on Lina's life insurance policy to repay Melbeck. Her suspicions allayed, Lina tells him that they will face the future together.

Question: Who writes a letter to Johnnie?
Answer: Lina