Step 1 - buy a cat, choose one from fat lines that enjoys its food where possible.
Step 2 - feed the dog (with the eating disorder) before the cat. Even though cat is over-weight cat will be so hungry that he will eat dog biscuits.
Step 3 - sit back and watch your dog gobble down his food. Should dog step away from a still full bowl, wait for cat to approach the bowl again, and then ask dog what happened to your dinner? Watch dog race back to dinner bowl and complete meal.
In all seriousness I can't believe this. I have one pet that is certified over-weight and one that is certified under. The under-weight one needs to always have food available as he grazes (we have tried the bowl down for 15mins then nothing but he lost a further 5kg - might have lost more but we switched methods) and the cat is so fat that he will eat dog biscuits. The only upside is that Sammy is finally eating because he's so competitive with the cat. He may not have any food drive, but he has very competitive nature...