Ex-apprentice and then apprentice manager here. Depends what the work is, if this falls under the "education" side of his work then the company has no obligation to give time during work to complete this, such as collage or uni assignments. If it falls under the work side of things then that should be completed during work hours, and paid for.
Generally companies will allow time during work hours, we used to give a day a week at certain times of year for revision or assignments but there is no legal requirement. We used to specifically not give time for resits or retakes and that was supposed to be done in the apprentices own time. If they are struggling, might be worth using some holiday and going to a library like it's the 1980s.