Should he choose marriage and stability or college, knowing that the girl is not waiting for him to complete his studies?

The Sharia encourages marriage for many purposes when one is capable, and people's circumstances in this regard may vary. Therefore, marriage is subject to the five legal rulings: obligatory, forbidden, recommended, disliked, and permissible. If marriage is obligatory for you, prioritize it over completing your education. We do not see a contradiction between them; rather, marriage may even help a student of knowledge in their studies. As for the prevalent love between genders in its known form, it is forbidden by Sharia due to the transgressions it entails. Therefore, you and she must repent from it. If your repentance is sincere, then propose to her. The servant's happiness in both worlds can only be attained through obedience to Allah, and disobedience is the cause of all distress and worry.