Jasmin, good morning.
(1) Your friend was not very courteous to say what he did.
(2) Nevertheless, some people might agree with him in his use of "so."
(3) According to what I have read, as...as is ALWAYS correct.
(4) But some people prefer so...as in negative contexts.
(5) Some writers have this theory: We are so accustomed to having as...as refer to affirmative sentences (Tom is as tall as Sue) that the negative idea comes across more strongly if we use so...as for negative contexts (Patricia is not so courteous as George).
Like you, I'm waiting to see what the experts have to say.