In my experience, the kind of behavior being exhibited by the unnamed person falls neatly into the category of Avoidance. It is not overtly malevolent. It is never premeditated. It is rooted in low self-esteem and a sense of powerlessness. Basically, it's a self-fulfilling prophesy that they are a schmuck.
This is very different than someone who is having a hard time in life and is overwhelmed by situations not of their making. That person will usually not enter into agreements that they know that they can not fulfill. They tend to withdraw and circle the wagons until the Indians leave.
That said, when dealing with someone like this, kid gloves are the worst way to go. Keeping it low key and trying to be sympathetic is simply enabling them to continue with the same behavior. You need to call them on it, and if that doesn't work, to do so publicly. Being understanding may seem to be the most compassionate thing to do, but it is really the most damaging by allowing them to continue. Ad hominems and vitriol are not necessary and are most probably counterproductive. In short, what Daniel has done so far is what is needed. The next step is name names, if for no other reason than to warn fellow forum members. This person needs a cold bucket of water thrown on them, and be told to grow up and cut it out. Granted, that isn't going to change them, but it could be the start that they need.