This minhag is common to Sefardim and Ashkenazim in EY. In America, as is well known, Ashkenazim do not have this minhag. While the gabbai may have overreacted, the guest yachid hamispallel has no right to do any public action in shul without the gabbai’s express permission. I would have spoken to the fellow after shul and explained this to him, but I still support the gabbai here. The guilty party here is the mispallel and not the gabbai.
One more note: The authentic Askenazi minhag is not to say barchu. In EY, the Ashkenazim picked it up from the Sefardim.