Is repentance sufficient for a man who exposed himself to two young girls, or is his act considered an ongoing sin due to the potential psychological impact on them in the future?

What your friend did is considered an abominable act and among the forbidden things, and it indicates a lack of modesty. The Messenger of Allah (peace be upon him) said: "Guard your private parts except from your wife or what your right hand possesses… for Allah is more deserving that one should be shy of Him." The door to repentance is open, no matter how great the sin. Your friend's remorse is the foundation of repentance, and he must add seeking forgiveness and resolving not to return to this act. For Allah, glory be to Him, said: "And Allah would not punish them while they seek forgiveness" [Quran 8:33], and He said: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" [Quran 39:53]. This sin is not considered continuous; rather, if he repents from it, Allah will accept his repentance.