Hi, and welcome to pets.ca!
First of all, I'm going to assume you're talking about dry food. Which means my next suggestion is going to be not to feed dry food
. Sounds like your friend's cat is overweight, and the best remedy for that would be to cut out the kibble and feed a good low-carb canned food. All of these diet, light, weightloss, hairball, and senior formulas are completely inappropriate foods for a carnivore (way too many carbohydrates - which is what makes cats fat in the first place). I suggest reading this website for more info on feline nutrition:
http://www.catinfo.org/
Oh, and Wellness is definitely better than Royal Canin, but most canned foods are better than even the best dry food.