What is the ruling on a woman who engaged in forbidden acts, such as kissing and embracing, with her beloved while fasting during the daytime in Ramadan, and what should she do to atone for her sin?

What your friend did is a sin that necessitates repentance to Allah. She must distance herself from this man and avoid being alone with him, due to the Prophet's (peace be upon him) saying: "No man is ever alone with a woman unless a mahram (unmarriageable kin) is with her," and "No man is ever alone with a woman but Satan is the third of them." Her fasting is valid, and she is not required to make up for it unless semen was discharged from her due to touching with desire, in which case it invalidates the fast and necessitates making it up.