Is what he is doing, informing a girl of his love for her with the intention of marrying her later, while not socializing with her except in the presence of colleagues and only for study matters, considered a sin against Allah, and what should he do?

We remind you of the words of Allah Almighty: "And let those who find not [the means for] marriage abstain [from sexual relations] until Allah enriches them from His bounty." Train yourself in chastity and seek help through fasting. Occupy yourself with permissible academic or athletic programs, and frequently read books that inspire hope and instill fear [of Allah's punishment]. Read about the horrors of the Day of Judgment, the bliss of the people of Paradise, and the torment of the people of Hell, for this suppresses desires.

A man's conversation with a woman is permissible if there is a need for it, and this includes informing her of his intention to marry her. However, beware of Satan, who uses women as snares to trap young men. Do not let your self-confidence lead you to excessive communication with her. Stay away from excessive communication with this girl, and distract yourself from thinking about her by engaging in beneficial activities. Keep her out of your thoughts until you are able to marry her. It is better to seek the help of your family to expedite your marriage to her, for the Prophet, peace and blessings be upon him, said: "Nothing is seen for two who love each other like marriage."