So that is how to create a single story: show a people as one thing, as only one thing, over and over again, and that is what they become.Adichie overestimates the power of media, underestimates human stupidity. Even when there are multitudes of different stories presenting different perspectives and facets, like on the internet, with Google and Wikipedia and armies of bloggers on either side of everything, many people just hear the story they want and shear away complexity, nuance, context.
The single story happens in people's heads, it's intellectual laziness. So yes, Adichie's roomate was a terrible person for making assumptions about what Africans must be like. Any group, Americans, Palestinians, blacks, Eskimos, gays, Aztecs, whatever: some are stupid, some are smart, some are heroes, some are bastards, etc. This should be blindingly obvious to anyone by the time they're a teenager. Stereotyping, thinking any group is a monolith that thinks with one mind, speaks with one voice, is immediately stupid, no research or travel required.