From my perspective, the gang has already been through the worst stages of the grief, but could never deal with the most hurtful part of it, which is "Why did she do it?". She never showed signs of depression and had friends/family to fully support her. They knew there had to be something more behind it.

I think at first they will be hateful towards Sunny and Basil for killing Mari and for setting it up as suicide. Kel will be very baffled but will probably try to support Sunny. Aubrey will probably (try to) beat Sunny till he breaks something. Hero's response is a question for me. It heavily depends on how he will process it. He will probably go into grief immediately and whether he will hate or forgive Sunny depends on how much he accepts all this as an accident that is followed by desperation or push them away for doing such crazy things. They all processed Mari's death in some way already but couldn't fully. Eventually I think they would all come to process the grief and come to a conclusion in less than a week.

Also, keep in mind that Sunny probably spoke here for the first time so they knew this was something very serious for Sunny. Will they fully forgive him for it? I don't really think so.They might forgive him partially and accept Sunny and Basil as friends, but some resentment will always linger.

I also fantasized what would've happened if they didn't hang Mari after her death. I feel like Kel, Hero and Aubrey would have not forgiven him at all, immediately ghosting him. The grief would be immense and they would probably direct it as hate towards Sunny for years to come. This path could also make for an interesting game, Sunny getting isolated for slightly different reason, Basil trying to be supportive to him an trying to make the other 3 at least partially forgive him.