This situation is known of, and was previously allowed, and the chances are that the acquaintance is of a certain age (without wishing to sound ageist probably 70 or over) Indeed the Staatsbürgerschaftsgesetz 1965 (repealed 1984/5) stated in Article 9 that a spouse could receive Austrian citizenship by marrying an Austrian (source: Stammfassung StbG 1965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. This legislation was however repealed in 1984/5 and the current version of the Staatsbürgerschaftsgesetz 1985 as amended does not contain such a provision. There was only a requirement for the marriage to be intact “am Tisch und im Bett“.