I would still give them those kinds of treats, I would just separate her while they get any special foods. No bread for her, but I think the vegetables and stuff sounds fine. Do they free range? I would make sure she was free ranging, or somewhere that she got eat lots of plants and bugs, and has a reason and the space to run around for exercise. It is my opinion that chickens should not have 24 hour access to a feeder, especially if they free range. Not only will they pick through it and only eat what they want, like the corn, they will eat less real food, like plants, seeds, and bugs because they can eat commercial grain, which really is not all that great for them. In the morning, I would put down grain in a pan or whatever for them, enough that each chicken will eat in twenty minutes, and separate the overweight hen with her own 1/4 of a cup ration. When everyone else is done eating, let her out with everyone else, with no grain sitting out. Do this again in the evening, and if you are able, in the middle of the day. This is assuming they free range or have a pasture where they can get real food in between.