What is the correct Islamic course of action towards a girl with whom I had a relationship and then left for the sake of Allah, and I have learned that she has deviated behaviorally? Should I advise her directly or warn her father, taking into consideration the consequences of each action?

You did well to end your relationship with that girl, for emotional relationships between young men and women are forbidden. You should choose a righteous woman from among your Mahrams or acquaintances to offer her advice and guide her to the path of chastity. If advice is not effective, and you are certain that informing her father will change her situation, then inform him; otherwise, do not. And preventing her from studying is obligatory if it prevents her from committing sins.