Is it permissible for my friend, a physical therapy student who has not yet graduated, to receive a salary for his work as a physical therapist in a center, given that the owner of the center and the patients are aware that he is still a student?

It is not permissible for a student to work as a doctor without a license. The law, which achieves benefit outwardly and inwardly, must be adhered to. For a student to present himself as a doctor is deception. He must be distinguished from a doctor, and patients must be informed that he is an intern and does not have the authority to diagnose and examine as a doctor does. All of this is forbidden deception, and the resulting salary is forbidden, and deception is one of the grave major sins.