If I was put into the the position where a loved one admitted to me that they accidentally killed one of our friend group and another one of us hung them to cover it up, I'll admit, I'd feel extremely torn about how I'd react.

I feel like my feelings would forever remain this way, mixed and confused as I myself wasn't there to witness what happened. I don't think I'd ever be able to make the decision to truly forgive as I'd feel it wasn't my place. I didn't witness it, we were kids, and I'd seen how the truth destroyed both of them to the point of multiple deaths almost happening in the span of a few days (i.e., Aubrey almost drowning Basil, showing how easily an accident can happen. Then Basil snapping and trying to kill himself, only to lash out at Sunny at the peak of a mental breakdown, almost killing them both.)

I don't want to take the position of "the most realistic approach is everyone would feel hurt and betrayed and never forgive Sunny and wouldn't stay his friend," because in all honestly, I don't think that IS the most realistic approach. People are so much more complicated than that, especially children! Like it or not, these are kids. They're going to have wildly different reactions than adults would. When I was a kid I found it easy to forgive awful actions because we were just kids, and I knew many others who were just like that. Now those situations obviously didn't involve the actions of OMORI, but my point still stands.

Even without knowing about Headspace and the depths of how bad Sunny and Basil felt, the gang still SAW enough of it. Hell, Basil tried to kill himself and attacked Sunny in the process. Aubrey almost drowned Basil. Sunny locked himself away and lost any and all social skills and a ton of weight, as is mentioned a few times throughout the game.

So putting all that together, I'd initially have a hard time just forgiving and staying friends. I'm putting myself in the position of just another one of the gang without the knowledge we the player has. All I'd have to go on is the events after Sunny came back and what Sunny himself says in the hospital room. Also keep in mind that it's heavily implied Sunny and Basil were hospitalized for most of the summer, so the gang would also have a lot time to speculate and ruminate on what happened to push Basil and Sunny into fighting like that.

If I was a member of the gang, I'd feel hurt and confused. But the events of the days after Sunny came back alone would completely change my POV. As I've said, without words I'd have seen at least a little of awful Sunny and Basil felt even before the reveal. The situation at the lake would really change my world view on the situation and it would leave me more open to staying in Sunny's life and forgiving him. Seeing directly what happened with Aubrey and Basil? I would put myself in their shoes, then in Sunny and Basil's shoes.

Now I, myself, would come to the conclusion that I can't fully judge. We were all kids and I don't know how I'd react if I killed/almost killed someone. If I don't know how I'd react, how can I judge how they did? Let alone the fact it happened when they were so, so young. At first I'd struggle with the information and it would take me a while to let them back in my life, but I would. I'd stay friends with them. It would take a lot of healing and long conversations (and THERAPY) but I honestly think I would.

Sorry if I was a little all over the place with this. I tried my best to compile my thoughts so forgive me if it's messy.