I think the main thing about "circles of grief" is not passing any grief or anxiety or burdens "in" -- that is, toward the center of the circle. It's being conscientious about where you lie (in your case, it sounds like there's a mom at the center of the ring, then her immediate family, then probably those close to her who aren't family, then acquaintances of the ailing person, and then friends of those on the inner rings, very roughly speaking).
So, as a person on the outer ring, what can you do?
You should listen, and try to understand your friend's needs. If his needs are food, then you provide food, if you can, and if he seems to just need a listening ear, listen. Ask what he needs, but be understanding if he says he doesn't need a thing (but also, weirdly, understanding of the fact that not saying anything is sometimes an indicator of great/aimless need).
I'm sorry you and your friend, and his mother, are going through this.