https://www.marxists.org/archive/lenin/works/1920/lwc/

Particularly the chapter where Lenin critiques the German communists- https://www.marxists.org/archive/lenin/works/1920/lwc/ch05.htm

I think your friend's main flaw is flattening what a bourgeois dictatorship looks versus a proletarian one. If everyone has access to literacy and education, the intelligentsia isn't a walled garden. I've yet to meet a Marxist who told anyone to read fewer books. The people I loan my books to are minimum wage workers without degrees, and I give them the books with supplemental podcasts so there's no barrier to understanding them. I want them to absorb and consider those ideas so that we can debate them in a union. I want that union to grow larger so that people like me have a voice in government. I want those unions to be united by a vanguard party so that we can maximise our power and use it effectively at any level it's challenged. That inherently represents my interests better than any existing party and my democratic participation in society is much greater than I currently have regardless of my demographics. That dictatorship of the proletariat is a transformative process, not swapping out one boss for another.

Attributes the fall of the USSR entirely to the democratic organization of the government.

Your friend should really read Blackshirts and Reds- https://welshundergroundnetwork.com/wp-content/uploads/2020/04/blackshirts-and-reds-by-michael-parenti.pdf

The flaw of their understanding of history is that it's undialectical. The Soviet Union somehow exists in a vacuum where they spontaneously make bad decisions because they're elitists until finally they shoot themselves with the hubris gun. If only Lenin had described communism as "Soviet power and the electrification of Russia", if only he hadn't made an immediate priority of universal literacy, maybe the peasants would have fixed everything instead of just making their grandson the first human in space.

Everything is an interdependent relationship between organism and environment. You can't study your own body unless you know how its systems work together and how it shapes/responds to your surroundings. You can't study the Soviets without also understanding the Germans, Americans, or Chinese. You can't understand Chinese development without seeing how they were shaped by the British, Soviets, Americans, and the modern global south. The Soviet Union existed under siege by the world's superpowers for the entirety of its history, with thousands of nuclear weapons aimed at it by the only country to ever use one in a war. They faced a war of extermination and lost 20 million people along with their industrial and agricultural base, fighting off the vast majority of the German army while the allies debated marching on Moscow after the war. Liberalisation was a regressive economic war imposed on them externally and after it your friend gets to pick which alternative Russia they prefer- pre-revolutionary imperial Russia or post-Soviet Putin's Russia. If neither of those are better than the Soviets, if the only existing global alternative to what the Soviets achieved is a dictatorship of the bourgeoisie we can all agree serves none of us, your friend is just debating which wheels and accessories they'd like to put on their grandma to make her a bike. I don't care about baseless idealistic speculation if I have tested theory which created two superpowers out of ruined countries. If the most marginalised person in society wrote that theory and it's still based on observation, it's just as valid and intellectual as a PhD writing it.