If you put the ability to make a safe decision that it's an appropriate time to fell a tree, under the tent of falling skills, where it belongs, then you can correctly attribute a lack of falling skills as a causal agent in the unfortunate death of this tree worker. Had the feller been competent, they would have declined to undertake felling operations in such close proximity to another tree with a worker in it.
mrtree is absolutely correct. Workers not involved in the actual felling operation (The climber in the adjacent tree obviously wasn't) must be at least 2 tree lengths away from the tree being felled. Workers involved in the felling operation (pulling tag lines, etc) must be at least 1.5 tree lengths away from the tree being felled, with preplanned escape routes and a means of communicating with the feller.