「いつの間(ま)にか彼女(かのじょ)はネイティブみたいに話(はな)せるようになった。」
"Without me realizing, her speaking has grown to that of a native."
[Since いつのまにか is not used when change is gradual, the sentence suggests that speaker has not seen her for a long time, and during that time she has become proficient with language]