Scientifically speaking, there is no reason why we should avoid eating fish and dairy together. And you can find many recipes that incorporate both together, like baked fish with a cream-based sauce.

Just make sure these foods are handled properly and safely before consumption, i.e the fish is fresh and cooked well, and the milk or dairy is consumed before their expiry date. Only in case of people who are actually allergic to fish or who are lactose intolerant will these foods create difficulty.