The harsh truth is that it's simple. The individual must burn more calories than they eat. If they do so, they will lose weight.
Obviously this can be achieved more than one way. Decreasing caloric intake. Increase caloric burn. Or some combination of both. Now, eating crap food makes reducing intake less likely.
One can reduce caloric intake by eating similar quantities of BETTER foods, though this still may be taking in too much food. Especially if one does not combine the dietary changes with increased exercise.
The current crop of diet drugs have a fascinating potential for those without the discipline to cut intake or increase phsyical activity. I don't know if they will have to remain on the drugs or if the weight will return quickly if they get off the drugs.