This is a significant problem, and you should not try to deal with it yourself. I suggest you find a therapist experienced with DID
and ask them for assistance.
To answer your question, it is not possible to "lock away" an alter personality forever when the patient is in the hands of an experienced and I mean experienced therapist. This takes careful handling by a skilled professional.
As for bringing your friend back, it is important that he has very little stress
and obviously learn how to control his anger. High stress situations make dissociation worse. If your friend's other side will talk to you, you could try to negotiate with him, and see if he will allow your friend to surface sometimes. However, it sounds like you will be punished if you tell your friend about this other side, and it also sounds like there is co-consciousness, meaning that the other side knows what is said to your friend, while your friend has no idea of the other side. This means the other side is stronger, so you must be very careful.
Understand this angry side was created to help protect the more vulnerable part of your friend. Make sure you don't antagonize this part and make sure he knows you have no intention of hurting any part of your friend.
As you can see, this is quite complicated, which is why I suggest that you get some council from an experienced and skilled professional. Please let me know if you have any other questions about this most distressing situation. I wish you well.