There were three fit hikers. It sounds to me like two of them stayed with the injured one while the third went down the mountain to make the first call. Then the caller returned to the group, and then the first two returned to base while the caller stayed with the injured man. There was talk of the two going back up with sleeping bags etc, but maybe the caller guy was able to get what was needed at the place he phoned from. If this is right, the two who left were not leaving the injured man alone. With hindsight it would have been better if at least one more person had stayed on the mountain, but they probably didn't think it was necessary.

Then the injured man got worse. The chap with him made another journey down to make a second phone call. At this point the injured man was left alone, but the caller didn't have much choice about it given the apparent escalation of the situation, and there now being no-one else on the mountain to leave with him.

After making the second call, the caller was unable to return. I'm guessing he was knackered on account of having travelled up and down twice and couldn't face a third ascent. He could have sent one of the other two, but by now that would have meant getting them out of bed. Probably he thought it unnecessary given the helicopter was on its way. He may not have appreciated the difficulty of getting the helicopter out, nor realised it wouldn't arrive for another four and a half hours.

This is my reading based solely on the article. The hikers behaviour doesn't sound like what people experienced in similar disasters would do, but nor was it completely callous. They just didn't expect the injured hiker to get worse during the night, and so thought one person left with him would be enough.