Deprogramming someone from anti-Stalin hysteria is difficult, because it’s so deep-seated. If you’re not careful, they’re going to think you’re doing something akin to Hitler apologia. I think the best you can do initially is to just try to convey that he was not that different from a lot of other strongmen leaders, and emphasize the absolute madness and chaos of the times he lived.

You’ll have to make the case simultaneously that the USSR was not the ultimate evil. That’s an easier task, because I think most people haven’t really been exposed to the arguments: that living standards improved dramatically through-out it’s entire history, only stagnating during the 70s and 80s (when the US was also stagnating). And that it did that despite starting basically from Zero in the 20s (really emphasize how poor and underdeveloped the USSR was in 1922), and then having to defend itself in a gargantuan, genocidal invasion from Nazis who killed 1 in every 6 people in the country.

If you compare to other strongmen leaders, who loot the country to live an ostentatious lifestyle, while running undeniably ineffective governments with stagnant economies, and then compare to the USSR and Stalin… you might plant the seed that maybe Stalin was actually earnestly trying to run a country well, and that he actually did a pretty good job considering the circumstances.