Oof, that's a tough situation.
Taking the bird from your dad and locking it up after it's already on him won't make the bird avoid your dad... it will make him avoid you. Think of it from the bird's perspective. He's there with his friend, enjoying a nice snack together, and every time you show up, it means the fun is over. I'd bite you, too.
Obviously your dad loves feeding the bird. Could he be convinced to use a substitute food? If your bird has an absolute favorite treat (one that's on the approved list of foods), such as sunflower seeds, maybe you could leave out a little airtight container, and when Dad eats, he could offer one of those instead of whatever he's eating himself. If the treats are right nearby, there's no extra work involved in getting them out, and that way they could still enjoy their treat time together, but not put your bird's health at risk.
I've had similar issues with my boyfriend, who is the biggest softie in the world when it comes to begging pets, and we literally keep a jar of birdseed on the coffee table for just these situations.
That said, the birds still do get their share of people food, and I've learned to pick my battles. I make sure Boyfriend knows what's toxic (chocolate, garlic, onions, avocado, etc) and let it slide with things that aren't necessarily healthy, but won't do any harm in small quantities. Bird wants his hamburger? Bird may have a piece of the bun or lettuce or tomato. The bird is eyeing his plate of garlicky pasta? We set aside a couple pieces of pasta with no sauce (Boo goes NUTS for plain spaghetti noodles).
You could even post a list on the fridge of foods that are toxic to birds. That way Dad would know that if it's not on the list, it gets a green light.
Cheese was the toughest one to deal with, because it's Boo's absolute favorite food (she came to us a cheese junkie), and I wasn't willing to risk crop impaction. I found the solution in this lovely stuff called "nut cheese." It's a vegan cheese alternative, and she doesn't know the difference. (NOTE: If you go looking for nut cheese, you have to vet the ingredient list carefully, many nut cheeses contain ingredients that are just as bad or worse for a bird than normal cheese).