No, no, no, mom is WRONG! Sorry to dispute mom, and I'm sure she means well. I know that used to be something that was said a lot....in fact, my (idiot...sorry) MIL still says "oh...all wet food will give them diarrhea", and I jsut want to smack her. No. Changing a cat to a new food may give it diarrhea and a food that is full of stuff that the cat is possibly allergic or intolerant too may give it diarrhea, but it's not that it is wet food. That is an old school way of thinking, it is simply not true.
Your cat is trying to eat what it knows is best for him, and hard kibble is not what is ideal for cats. It was made to be more convenient for humans, but it is hardly ideal for cats. It does not have much moisture at all, leaving cats dehydrated. Wet food is 78% water, MUCH better for keeping cats, who are not big water drinkers, getting enough liquid in their diets, keeping things flushed out, and preventing UTI's, crystals, blockage, and other health issues.
Your mom is likely not going to be receptive to you telling her she is wrong, but you may want to "gently" try to let her know that things have changed and is now known that wet food is bets, and explain about the dry food having no water, canned having lots, cats having urinary issues and needing the extra liquid to prevent those issues down the road....and hopefully you can convince her that you want to do is best for your cats that you love so dearly. Let her know that it will SAVE you money and heartache down the road because your cat likely wont' have those issues if it eats a proper diet. Even if you can't afford one of the more expensive canned foods, a cheaper one is better than any dry food, in my opinion, and many peoples opinion.
Let your babies eat the food that they know is best for them. Try to convince mom that you want what is best for your cats, that you love them and want what is best and that thinking is so different now about dry food being better. I wish you the best of luck in trying to convince her otherwise. Your kitty does not want the dry food that leaves him feeling unsatisfied, he wants the good stuff. Good luck!