The only groundschool I had for my PPL was the radio course, which was money well spent. But that was structured, booked in advance, and I knew exactly what was occurring.
Quick, friendly chats over a brew in the cafe are NOT groundschool, and I think your dad is right; they're trying to take advantage of a student who doesn't know any better.
Keep it polite, but just mention to them that you're not comfortable with being charged for what was never anything other than a friendly chat, and certainly wasn't pre-notified as groundschool. Don't feel bullied into not saying anything. They're almost certainly not going to jeopardise the potential revenue from a full PPL course over it.