You're right, that argument is bullshit. People only believe it was necessary to drop the atomic bombs because of chauvinism, US propaganda, and racism. I've had these arguments before, and the other side always falls back on national myths about Japan's unwillingness to surrender that we were taught in school or by the media, or just absorbing the commonly held “wisdom” about how the war ended. There's a lot that we aren't taught about the bombings, so I'll try to show the receipts to convince your friend that you're right.

What is most telling, I think, is that the Pacific Front’s admirals and other general-level officers throughout US high command opposed Truman's decision to use the bombs. These admirals and generals are on record stating that the atomic bombings of Hiroshima and Nagasaki were of no strategic value, were immoral/unethical, and did nothing to hasten Japan’s surrender (which was inevitable by that point). The bombings were even condemned by Truman's Chief of Staff Leahy. Dropping the atomic bombs was a political move, not a military one, and it was opposed by some of the highest officers of the US military.

Seven of the United States’ eight five-star Army and Navy officers in 1945 agreed with the Navy’s vitriolic assessment. Generals Dwight Eisenhower, Douglas MacArthur and Henry “Hap” Arnold and Admirals William Leahy, Chester Nimitz, Ernest King, and William Halsey are on record stating that the atomic bombs were either militarily unnecessary, morally reprehensible, or both.

https://www.latimes.com/opinion/story/2020-08-05/hiroshima-anniversary-japan-atomic-bombs

You can ask your friend whether they know the strategic situation of the Pacific in 1945 better than Nimitz, Halsey, or MacArthur. We can look at the situation materially and find some answers there. Japan had lost its occupied territories, meaning it was starved of oil and steel. You need massive and continuous inputs of energy and raw materials to wage a conventional war, and Japan was being strangled. The USSR liberated Manchuria, and Japan was surrounded. Japan's air forces were depleted, so the US controlled the skies. It was only a matter of time until Japan surrendered.

That's not to say Japan didn't still have enough men or materiel to inflict massive casualties but chauvinists want to act like grandmas and babies were getting ready to throw grenades at US troops. This is the racism I'm talking about, assuming that Japanese civilians were this mass of fanatical bushido blades that could not be reasoned with. The idea is that Japanese people, not just the imperial soldiers, were part of a death cult that was fully committed to fighting to the last man, woman, and child for the emperor. Civilians fought back in many different countries in the war, including Germany, but people don't make sweeping statements about how they all think alike or you can't trust them, so we better kill an entire city.

There are interviews online with US bomber crew members who bailed out over Germany, which I think help to underscore how there's a racist double standard. In one instance, the crewman was grabbed by German villagers who were very pissed off, because he had just got done dropping bombs on them. The villagers had a noose and they were getting ready to lynch him, but were actually stopped by a German officer. In another instance, the crewman fell partway down a slope after cutting off his parachute. One of his buddies who survived bailing out had landed at the base of the slope, in a farm. A German farmer was there, liming the soil. The crewman giving the interview described how the German farmer, a civilian, (CW violence and murder)

The interviews are fairly recent, but I'd probably have to do some digging to find the video again, it's on YouTube. Germany also forced children and old men to fight when the situation was hopeless and Germany was being invaded. Clearly at least some German civilians were willing to kill enemy combatants, viciously and brutally, and they mobilized civilians who were barely fit for combat, but no one uses that to justify the firebombing of Dresden or generally the strategic bombing of highly populated areas in Germany. If civilians in Allied countries fought back, they're called partisans or Resistance fighters. There's a double standard. I think it's akin to the US/Israel claiming that the civilians they kill are enemy combatants, which is only possible if you dehumanize the victim. Racism is what allows people to believe that Japan was a hive-mind that would fight to the death and never surrender.

People also point to the Japanese soldier who kept fighting after the war ended because he refused to believe the emperor would ever surrender. Well, what did the rest of the Japanese military do? Did they keep fighting too? To be clear, was he a soldier or a civilian? 1.6 million soldiers surrendered when they were told to, and peace talks were already being considered by both sides. Also, Imperial Japanese Army training was absolutely brutal, with absolute obedience being imposed through beatings. You can't base your appraisal of the entire civilian population of Japan on the behavior of one brainwormed Japanese soldier. The civilians that were mass murdered in Hiroshima and Nagasaki were innocent, and they did not pose any threat.

Peace talks were already on the table, but the US wanted the emperor to abdicate. As I understand it, American Anthropologist Ruth Benedict convinced the Truman administration to drop the abdication demand, because the psychological/cultural damage of deposing the emperor would be so severe that it would likely cause problems for the US, which wanted to maintain a hegemonic peace. Well, they kept the emperor and today there are still US military bases in Japan.

Hopefully that's enough to get your friend to budge. I think the myth of the good war is what makes this hard, it's a main part of the American Civil Religion. But if the roles were reversed and the Axis won, the Nazis today would still be saying the decision to drop the atomic bomb was a necessary way to save hundreds of thousands of lives. It should be remembered as a war crime.