This situation applied previously, but no longer does. The chances are that your acquaintance married prior to 1985. The Staatsbürgerschaftsgesetz 1965 (repealed 1984/5) stated in Article 9 that a spouse could receive Austrian citizenship by marrying an Austrian (source: Stammfassung StbG 1965 https://www.ris.bka.gv.at/…/1965_250_0/1965_250_0.pdf) without any mention of the need to give up their British citizenship.There was only a requirement for the marriage to be intact “am Tisch und im Bett“. This legislation was however repealed in 1984/5 and the current version of the Staatsbürgerschaftsgesetz 1985 does not contain such a provision.
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