Since A and C could see B below it stands to reason that he could see them and if they had a problem. At 6 m he would have a better opportunity to to help somebody unable to establish buoyancy on the surface causing them to sink. If they were not visible to him he should have surfaced to check on them and if he chose drop back down to complete the stop. Maybe in your world there is no safety stop on a non-deco dive but in my world there sure as hell is.Diver B should not have split from the group all 3 should have stayed together, he made an unnecessary stop. If I was a cynic I might even think the pony lead to a bad decision.