WHY WASNT DUNNIT WITH THEM AT THE START

I don't understand why Dunnit wasn't with them in the cabin at the start of the movie. at first it seemed like Dunnit died before the movie. but then henry found him for help.
if they loved him so much why wasn't he with them as a group in the cabin. it just makes no sense. they didnt know he had Leukemia obviously.
if Dunnit was there from the start, the other two friends might've still been alive.