What is the expiation for a mature girl who kissed a foreign young man and then repented?

It is obligatory to repent and turn to Allah, and not to return to what was done. There is no specific expiation for this act, but it is among the sins that are expiated by performing good deeds, as Allah the Almighty said: "And establish prayer at the two ends of the day and at the approach of night. Indeed, good deeds do away with evil deeds. That is a reminder for those who remember." [Hud:114].