First, you have your heart in the right place to try to help. But also, do not try to take too much on yourself, okay? Some things you need a specialized therapist for that.
Second, it's commonplace that the various identities within a DID system have issues figuring out if they are real or not, if the others are real or not, or if themselves exist for real. (Yeah I know, there is no logic behind asking the question "Am I real?" because if you are not real, who is asking the question?

) When new identities are created to manage the everyday life, and the previous ones enter dormancy, it can lead to a lot of inner crisis, questionning towards one's identity and reality and existence, and so on. It's not fun.

Third, and I know it's difficult to apprehend this (especially when you are the DID person yourself), but, all the identities/alters of the DID person
are the same person. Just like your left foot and right hand are the same person - just different parts of the same person. In DID the different parts take different names, aspects, and do not recognize each-others as "one and the same". The other parts can feel like complete strangers coming from the outside. They can even be hostile towards each-others sometimes! This is because of the "dissociative" aspects of the disorder. The traumas were so bad, the different memories cannot "integrate" and recognize each-others as the same "me". The different brain areas develop with very poor connexion between each-others, and each of them becomes a different "me" and sees the others as "not-me".
Technically the different alters/identities you interact with, are the same person. You just went from "talking to the hand" to "talking to the foot" so to speak. And because they are not properly connected to each-other, they only see their differences and not how similar they are. The foot thinks the hand is fake because hey, a foot cannot grab things with an opposable thumb, which means that nobody can, so anyone claiming they have an opposable thumb is fake, right? Or simply, the hand is not the foot, only the foot exists, so the hand must be fake.
It takes a lot of time and a specialized therapist to help the different alters, realize they are part of the same whole.
You will find ressources about DID in the "ressources" thread I linked in my signature (coz I'm lazy about fishing it every time I want to share it xD ) some of them are books explaining DID, which might be interesting for you in order to understand how your friend functions. Others are self-help books such as "Coping with trauma related dissociation" which can be helpful for your friend. (Assuming it's DID and not a fudgeton of comorbid disorders giving a similar output but needing a different treatment than DID and having a completely different mechanism inside the brain, of course.)
You cannot be your friend's therapist. But there is something super duper important you can do for him: help him built a safe attachment with you. DID people often start life with a disorganized attachment style (different alters have different attachment styles and overall it's a complete mess) and a safe attachment helps stabilizing the whole system (a system is the totality of alters, parts, fragments... composing a dissociated person) which is like, half the therapy. A good place to start learning about attachment is Wikipedia
https://en.wikipedia.org/wiki/Attachment_theory but you'll find books and papers on the subject through Google Scholar and others.
I hope other people on this board will be able to give you answers that will be helpful too!