There is no sin upon her, but she must make up a day in place of it; because voluntary fasting, once initiated, becomes obligatory; as it was reported that the Prophet Muhammad (peace be upon him) said: "If one of you is invited, he should respond. If he is fasting, he should pray (i.e., make supplication), and if he is not fasting, he should eat," in Sahih Muslim 2: 1054. If breaking the fast were permissible, it would have been better to break the fast to respond to the invitation, which is the Sunnah. And Aisha (may Allah be pleased with her) said: "Food was gifted to me and Hafsa while we were fasting, so we broke our fast. Then the Messenger of Allah (peace be upon him) entered, and we said to him: O Messenger of Allah, we were gifted a gift that we desired, so we broke our fast. The Messenger of Allah (peace be upon him) said: There is no harm upon you; fast another day in place of it," in Sunan Abu Dawood 2: 330, and Sahih Ibn Hibban 8: 284. And Ibn Sirin fasted on the Day of Arafah and became extremely thirsty, so he broke his fast. He asked several companions of the Prophet (peace be upon him), and they instructed him to make up a day in place of it, in Musannaf Ibn Abi Shaybah 2: 290, and its chain of narration is on the condition of the two Shaykhs except for Al-Taymi, as the four authors narrated from him, and Ibn Saad, Ibn Sufyan, and Al-Daraqutni authenticated him as mentioned in Al-Jawhar Al-Naqi 1: 315. See: I'laa Al-Sunan 9: 160.