There can be several different triggers for IBD, so in order to find the right canned food you need to figure out what's causing it in the cat. It can be a grain intolerance from dry food, a protein intolerance to fish or beef, fat content, or some other ingredient in the processed food. That's why the best route to go is a raw diet, because you're pretty much eliminating all the possible causes except for a chicken intolerance (which is not common).
If raw is simply not an option, I would suggest trying out a limited ingredient canned food like Dick Van Patten's Natural Balance Venison and Pea, or Duck & Pea and see if it clears up. Nature's variety instinct also has a grain free Rabbit formula that works well with these types of issues. I'm not sure what sort of budget they're dealing with, but it would actually be cheaper to feed raw than these foods, if they are willing.
Supplementing the diet with a probiotic and/or a fiber suppl. like psyllium can also clear up IBD. Make sure that the new foods are given time to work--IBD doesn't just clear up magically as soon as you start a different diet--it can take a week or two as the cat's GI system adjusts. Probiotics can help a great deal with speeding up this process.
Since grain intolerance is a common cause of IBD, and even lower carb dry foods can keep blood sugar high, I would definitely not feed them dry food. However, make sure that they are home testing and lowering the dose if they are changing to a low carb raw or canned diet, or a dangerous hypoglycemic incident can occur.
