Hmm, if this is how your friend is, then I would say that you should first start with the stuff that even a liberal might be willing to admit (like the supposedly executed North Koreans that miraculously show up alive, as reported by even liberal news outlets); by contrast, starting out with "the DPRK is actually democratic" will be met with their brains shutting out the information because the propaganda is so deeply against such a narrative that they will ignore reality, so you should start with the stuff even anti-communists will admit is false or exaggerated. Also, I do not have sources, but it seems that Cowbee has a lot of sources that you can check out.

Honestly, how I got out of the "DPRK is full of evil yellow men" phase was seeing a bunch of "evidence" on the DPRK not add up, and the ridiculous levels to which news outlets will follow this narrative of "savage men" became so absurd and the other side (communism, and more specifically, Marxism-Leninism) actually explained the DPRK in such a better way that I had no choice but to admit that the DPRK is actually really cool. If your friend is even willing to have an open mind, then their propagandized perspective must be picked at from all angles.