My shul davens "standard" Nusach Sfard. During Shabbat mincha, the sha''tz was a Chaba''d chasid. While we were returning the Torah, the sha''tz started saying Kaddish. (Nusach Chaba''d/Ari only says one paragraph: Yehallelu.) Also, as it was R' Schneerson's yahrtzeit last Shabbat, the sha''tz also did not say Tzidkatcha Tzedek.
After he finished davening, the gabbai asked him why he didn't follow minhag hamakom (the minhag of the shul that he is in, now). He responded that Nusach Ari is the only "authentic" nusach, and that Chaba''d never follows minhag hamakom.
I know nothing about this aspect of Chaba''d practice, and the gabbai was surprised to hear this. (As a matter of fact, he was annoyed.) Can someone verify if in fact this is Chaba''d "policy", or if this person was misinformed?