Just FYI students have a whole year of animal nutrition. The use of corn and grain based products is all based on OPPINION not fact. Vets don't see a problem feeding corn, and there really is no problem, other than the fact that a cat would not be eating corn in the wild. There again, cats don't live in the wild either. It's a whole other arguement in itself. Until anyone actually GOES to vet school, no one really will know what education a vet has. You are all so eager to say a vet doesn't know what they are talking about, when neither do you.
That being said, feeding less dry WOULD help a cat lose weight. Free feeding is what causes weight problems in cats. The cat eats a whole bowl, the owner fills it up. If your mom measured the cats food, like gave the cat 1.5cups a day, she would see a weight difference rather than free feeding.
However, I am more of a wet food fan. My cat who is a 9lb cat was on Fancy Feast for a while [it's all he could eat] he was eating roughly 3 cans a day, sometimes only 2 and he maintained his weight. So overall your mom shouldn't be feeding tons of wet food, and if she isn't over feeding the cat should no be overweight on a wet diet.