именно не друг, а знакомая

GoldenFish: She's just an acquaintance of mine. When you have more than one kind of person in your life, i.e. friends, siblings, cousins, professors, you avoid the pattern MY... because it sounds like you only have one.

Oh, really?...
Thank you, it's the first time I've heard this rule)
So, if I understand you right it's grammatically correct, but it sounds a little bit strange for native speaker? I mean - not idiomatic). Yes?

Correct. That's why you often hear people say "a professor of mine" because if they say "my professor" it sounds like they just have one.

Спасибо Вам, Uly)