But she also said she can't afford to feed mostly canned food. I think increasing the wet food and decreasing the dry can help your kitty lose weight even if the cat has some dry food to nibble on because some amount of dry food will be decreased and replaced by the wet. It's almost impossible to completely control how much your cats are eating in a multi-cat environment but you can do your best. As far as feeding cats in separate rooms, you don't have to complicate your life like that if it's too difficult. Use your own best judgement and be generous with wet food and limit the dry. But remember too many calories can come from any food, it's just that wet food has less of it. Just figure how much each cat needs and limit the food left out for all cats. With some competition it should be somewhat harder to overeat because at some point if your cat starts gorging and the other cats aren't getting enough, aren't they going to get hungry enough to eat more so less becomes available to your chubby kitty? If you're leaving significantly more out for all your cats than they all need then it's time to limit the quantity.
I don't reccommend prescription weight control diets. Hill's Pet Nutrition dominates the veterinary community but I don't believe those foods are very high quality and I tend to disagree with vets a lot when they push those Hill's diets stocked on their shelves and don't give you access to alternatives. Plus many weight control diets are even higher in carbohydrates and excess carbohydrates often lead to obesity. Good luck.
I think most indoor cats are a little overweight and I accept it because it's difficult to completely control. Just as long as they don't become obese. 14 pounds is a bit much for an average cat. If you can get the cat down to 12 or 13 pounds, and see that it doesn't gain weight, that's quite an accomplishment. That's what my Spotty weighs. He will always be a little heavy.