Is it permissible to continue sitting with a girl he intends to propose to after he finishes his studies, knowing that transgressions occurred between them and they repented from them, and is this sitting unlawful (haram)?

Sincere repentance requires three conditions: abstaining from the sin, regretting it, and resolving not to return to it. The engagement does not justify the fiancé sitting with his fiancée because she is still a stranger to him, and this is not permissible except after the marriage contract is completed. One must fear Allah and keep away from the fiancée until the marriage is concluded, for the Prophet, peace and blessings be upon him, said: "There is nothing better for two who love each other than marriage."