What should we do if students leave during the year meaning that a group for the Component 03 (Team Project) has only one member?

If the Team Project has already been completed, then this can be submitted even though only one student will be entered.

If the Team Project has not yet been completed, then ideally the remaining student should join a different group. If this is not possible then you should arrange for other students, who are not taking the qualification, to work with the student on the project. Collaboration is a compulsory element of the Team Project, so the student has to work with other students in order to complete it.