I doubt you'll get a decent main course haggis, or indeed your guest would thank you!
So we need to look for places that include haggis as a starter with neeps and tatties obviously! And then go for a classic fish and chips.
Guild of Forresters at the far end of Porty from Seaton Sands will be very difficult to beat. Prices are reasonable. Booking at the weekend is necessary and even at other times is advisable.
After your meal if the weather is even reasonable go through the back to beach huts for a few drinks then leave and walk along the Prom ... and if you go far enough another couple of drinks in the very friendly Rockville.
Your German friends will be over awed. My Ozzy pals were last week!