We should always be as proactive as we can be at work, especially when we are just beginning to learn the task, but never to the extent of doing someone else's job. This is what happened to the employee in this story, and I think it's also what got him into a very difficult situation.
Ineffective managers base their opinions on vibes, and I'd say even effective managers do that sometimes; after all, they are only human (even if they don't appear human at all). And if you used to give a really friendly, cooperative, proactive vibe before (because you were doing two people's worth of work), they are going to notice the energy shift when that's gone.
In my experience, it can be hard to explain to management that you didn't get less cooperative; you just can't keep working twice as hard.
These situations are messy and hard to explain. After all, they're often mixed with plenty of manipulation and blurred lines. People who take advantage of you don't do it upfront; they hide their tracks, so good luck explaining that to management without sounding bitter and uncooperative.
I hate that the only solution people on the internet offer for every work problem is: get another one. Of course, that's not as easy as they make it seem. In this economy? We have to find ways of solving issues first. At least we should try.
If I were him, I would politely say (and prove) that I was doing double the work, and ask whether my compensation should change so I can go back to doing that. I have a feeling management would tell me to stick to my 'low energy'.
This is why we should never normalize doing so much! Managers get too used to it.