The "negation = Genitive" rule cannot be extrapolated too far. It works only when the positive sentence used Accusative. Otherwise, the case just stays. (which kinda means that it works also when the positive sentence used Genitive :P)
Znajomy is an adjective with the meaning 'familiar'. But it most commonly functions as a noun ("a familiar one" = an acquaintance, a friend). Przyjaciel has a typical noun declension pattern, whereas znajomy has a typical adjective declension pattern.