We can't tell you anything about your friend as we aren't professionals here, but we can tell you how it is for us.

Many years ago, I would have said I had a relatively happy childhood. I had no memories of anything bad, even though I didn't really have any memory of my life. And while I was denying the existence of anything bad from my past, the people who abused me as a child were still doing stuff. I wasn't aware enough to know anything was going on. That's how far my brain went to survive and protect me.

At the same time, I denied (due to being unaware) that I said or did things that I didn't remember. I just thought people were tricking me.

The thing with DID is that the brain is about the business of protecting self from anything bad. And denial is a pretty normal way of doing that, imo.

Best of luck with your friend. I hope he is able to get some good support and help.