The Best Man

Childhood friends Olly Pickering and Murray go their separate ways when Olly goes to university. While there, Olly meets James. Murray vomits all over James, creating instant enmity between them.
Olly writes the two first chapters of a novel and an editor pays him $50,000 to finish it. But he gets writer's block, so loses all his money. He finds a job in London as assistant to Dana, an editor of self-help books for women.
James is going to marry in London and throws a party which he invites Olly to. Olly arrives late and gets covered in pigeon droppings in Trafalgar Square. He changes into some women's trousers. James wants Olly to be his best man and write a funny speech.
Olly feels out of place at the party. He is told off by Becka, the rightful owner of his trousers, and to Sarah without knowing she is James's girlfriend. While at the party, Olly meets, and immediately falls in love with Sarah while standing out on the balcony, but James thinks he has fallen in love with Becka.
Stan gives Olly a hard-core magazine for Murray.
Harry, Tania and barrister Graham discuss the matter in the pub. Murray will see to it that the wedding is cancelled.
Murray phones Sarah on Olly's behalf and they fix a date to see a film. Murray makes Sarah believe James is still a womanizer. Murray also sets Olly up with Sarah in Selfridges to see to the wedding list. He also phones James several times from Stan's porn shop, and appears at Becka and Sarah's flat saying that James has decided to sell it.
Back at Selfridges, Olly watches in amazement how Dana tries on a wedding dress although she is supposed to be already married and in a meeting. Dana gives Olly the sack next day. Graham talks to her trying to convince her to give Olly his job back. He doesn't convince her, but he ends up with a date with her.
Sarah finds some unknown red knickers that Murray had hidden to frame James, and she find the porn shop phone number in James's mobile phone, so she calls the wedding off.
Murray gives a surprise party for Olly with everyone there except James. Murray admits he's plotted everything and Olly writes a letter to Sarah on James's behalf. James is going to pretend that he is leaving for New York broken-hearted. Sarah falls for this and says she will marry him. Olly is left thinking he has done the right thing.
Murray tells Olly that James has shagged somebody while already in a relationship with Sarah, but he doesn't believe it.
Olly goes to James's stag party, which is a bit wild, with two strippers dressed as policewomen. James admits that he has been unfaithful to Sarah, because she can't satisfy him completely, although he wants to marry her as her family has money and connections, and she is pretty. Olly wants to tell Sarah, but James handcuffs him to a pillar.
When he gets free, he goes to the wedding place, preventing another wedding from happening while on his way. Murray is making a fool of himself trying to earn some time. Olly finally arrives, and with the help of Becka, who says that James has been unfaithful with his secretary and she blames another girl. Finally, Sarah calls off the wedding.
Six months later, Becka and Murray get married and Olly and Sarah end up together. Olly returns to writing and he produces a best-seller.

Question: Who does Olly write a letter to on James's behalf?
Answer: Olly writes a letter to Sarah on James's behalf.