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.
As long as your dad proves they are for personal use (if questioned) then he's fine.