"He is not my friend, he is my acquaintance."
Translation:On nie jest moim przyjacielem, on jest moim znajomym.
Okay, it seems that today a lot of people have problems with negations ;) Well, it's a very common misconception.
The rule is NOT "negation = dopełniacz (Genitive)".
The rule is "negated Accusative = Genitive".
Every other case, when negated, stays the same anyway. As Accusative is very widely used, people see that a lot and start thinking that every negation takes Genitive. But it does not.
Well, "best friend" is "najlepszy przyjaciel", so here "najlepszym przyjacielem".
On one hand, you are right. But... your sentence just seems surprising. "kolega" on its own is pretty vague, it may be someone not really important for you, just some guy you know. It just seems strange to make him even less important by saying "znajomy". Just seems not worth saying.
Still... we can't say that it's just wrong. I guess it won't hurt to accept it.
This type of question is not real, it is more like a test in grammar. Fine if you enjoy grammar, but where can we practice it for any who needs practice to build recall? I have no use for it here in Poland, and i would not on the internet. Od czasu do czasu, yes, so the grammar stays. This, no.