As long as your dad proves they are for personal use (if questioned) then he's fine.
Your dad should, pay by card. Paying by card is better, means he hasn't gone over with a bundle of cash someone's given him.
Keep all receipts.
If he is pulled up on return & questioned be honest with the amount he's bought. If he lies then they'll be suspicious. I bought 3k of tobacco & was honest when asked & they counted it.
No cigarettes as payment. Don't say 200 are for Fred who drove us to airport, that is a payment.
Typical questions are, do you work, how many a week/day do you smoke, when were you last away.
As long as your dad proves they are for personal use then he'll be fine. Technically it's free trade & there's nothing they can do, but their job is to catch the fag runners.