He has every right to choose not to eat the Halloween candy, but the fact that he wants his girlfriend to waste food because it goes against his diet is ludicrous. If you don't have enough self-control to refrain from eating candy when it's in your house, you might not be the healthy person that you claim to be.
There are some foods that I cannot buy because I can't control myself around them. I know that if I buy a box of Cinnamon Toast Crunch or Lucky Charms, I will eat the whole box within 48 hours. I love sugary cereal so much! In college, I ate like 3 bowls of these cereals every Saturday morning. No wonder I gained the freshman 15! These days, I cannot keep Diet Coke in my house because I know that if it's there, I will drink it, no matter how much I have already had that day. These are things I know to be true about myself, but I would never tell my roommates or my girlfriend that they can't keep things in the fridge that I can't control myself around. I can control myself enough not to eat other people's food, and I am not controlling enough to tell others what they can and cannot eat. After all, I am no health guru. Who am I to say that somebody else eats unhealthily? I am not a doctor or a nutritionist!
My girlfriend is a vegetarian, but she would never say that my eating meat is "disrespectful" to her vegetarian lifestyle. I know that there are vegetarians and vegans who wouldn't date a meat eater, and that's okay with me! What is wrong is springing the idea that your partner needs to eat just like you onto them in the middle of your relationship, which is what this guy did.