Is the lapse that my friend committed by engaging in zina (unlawful sexual intercourse) during the daytime in Ramadan with a non-Muslim young man, when she was twenty years old, forgivable, and what should she do to be forgiven now that she is twenty-nine?

Adultery is a major sin, and its gravity increases if committed with a non-Muslim during the daytime in Ramadan (definition). However, Allah opens the door of repentance for whoever regrets, resolves not to return to it, and increases in good deeds.

Whoever commits adultery during the daytime in Ramadan must repent and make up for that day. The severe expiation is to free a slave; if one cannot, then to fast for two consecutive months; if one cannot, then to feed sixty poor people. And whoever delays making up the fast must pay an expiation for the delay, which is to feed one poor person (half a saa' of the staple food of the land).