Your father's assessment of the goals of the Nazis is not a very accurate one. German war goals were much more than "loot and plunder" and they could be plenty destructive when it came to achieving victory.

The Germans did not invest heavily in nuclear technology during the war, and what they did invest in was reactor research, not bomb research. The reason for this is because they didn't think that nuclear weapons were very likely to pay off for the war. In the early 1940s, the atomic bomb was seen as a huge technological gamble. It was theoretically possible on paper (though even calculating that was non-trivial), but producing the fuel for it (enriched uranium or plutonium) was monumentally difficult and expensive, and in no sense assured to work. The Germans prioritized other projects. The Americans and British prioritized the atomic bomb, in part because they thought it was easier to make than it turned out to be (it cost around 5X more than they expected it to).

To give you an idea of the scale of a wartime atomic bomb project, the successful Manhattan Project employed around 500,000 people (a little under 1% of the entire US civilian labor force at the time) at dozens of sites across the country, spent $2 billion (1945 USD), and barely managed to produce three atomic bombs by the summer of 1945.

There was a thread on this very recently, with links to other threads, for more details. Or you can ask here.

the short answer is that while germany did certainly desire an atomic weapon and made the opening industrial and scientific moves to develop one, it never really blossomed into a serious effort with the enormous government backing that the successful Allied effort had for a variety of reasons

the Germans not only lacked the resources (be they industrial capacity, scientific knowledge, or human capital) that the Allies had access to, but also politicized science (particularly physics, with its many Jewish contributors) to such an extreme degree that it crippled their ability to perform the research necessary to bring an atomic weapon to fruition

the Anglo-American Manhattan Project employed many of the most brilliant scientists (including Doctor Emmet Brown if Back to the Future is to be believed) from the United States, Commonwealth, and continental Europe and was bankrolled by the combined material and financial might of the Allied powers

even then, it took the Allies till near the end of the war to have a usable nuclear weapon

that said I personally have little doubt that Hitler would have hesitated to use the bomb on either enemy population centers or military targets (be they tactical or strategic), particularly when things started going unfavorably for the German war effort in '43 and '44

it's also important to note that Hitler had ordered the razing of Paris and many of her landmarks when it became clear that its liberation was at hand but thankfully, Von Choltitz, the German military governor of Paris, refused the order on the grounds that it was utterly pointless from a military perspective and further was totally shitballs insane