Does what occurred between the young man and the young woman—seeing, touching, squeezing the breasts, and kissing, without penetration—constitute zina (unlawful sexual intercourse) necessitating kaffarah (expiation)?

What your friend did is forbidden (haram) and constitutes minor fornication, based on the Prophet, peace and blessings be upon him, who said: "Indeed, the son of Adam has been allotted his share of adultery, and he will inevitably realize it. The adultery of the eyes is looking, and the adultery of the tongue is speaking, and the adultery of the hands is touching, and the adultery of the feet is walking, and the heart yearns and desires, and the private parts affirm or deny that." He must repent, seek forgiveness, and not despair of Allah's mercy, and increase in righteous deeds, for Allah Almighty says: (Indeed, good deeds do away with evil deeds).