How does one who caused a girl to stray into watching pornography expiate his sin, and is he accountable for what she might do in the future due to his influence on her?

Sincere repentance, by the grace of Allah, erases sins. Your act of sending pornographic images to the girl is a grave evil. However, as long as you have repented sincerely, Allah accepts repentance and forgives sins, for He says: (And indeed, I am the All-Forgiving to those who repent, believe, and do righteous deeds, and then are guided) [Quran 20:82]. And because the Prophet, peace and blessings be upon him, said: "Indeed, when a servant acknowledges a sin, then repents, Allah accepts his repentance."

Since you were the cause of her being exposed to those images, one who leads to evil is like the one who commits it, as Allah Almighty says: (And do not cooperate in sin and aggression) [Quran 5:2]. However, sincere repentance erases that, and you will not be harmed by what she did after your repentance and your effort to clarify the truth to her.