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is of a certain age (without wishing to sound ageist probably 70 or over) Indeed the Staatsbürgerschaftsgesetz 1965 (repealed 1984/5) stated in Article 9 that a spouse could receive Austrian citizenship by marrying an Austrian (source: Stammfassung StbG 1965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 This legislation was however repealed in 1984/5 and the current version of the Staatsbürgerschaftsgesetz 1985 as amended does not contain such a provision.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“.
Will you have dual citizenship?
No. To gain Austrian citizenship will require me to give up British citizenship. The claim about “revoke and resume”, which Britain allows – i.e. to revoke your British citizenship to take another citizenship and then resume your British citizenship (at considerable cost) is not compatible with holding Austrian citizenship. Part of the reason for hitherto not taking Austrian citizenship hinged on the lack of a necessity to do so as well as the fact that prior to the UK Referendum on EU Membership, there was no need to do so, given that British citizenship entitled me to reside and work in Austria, and taken Austrian citizenship was contingent on giving up British citizenship. I am aware of people claiming that Britons in Austria may have used “revoke and resume” to obtain Austrian citizenship and then retake British citizenship , but technically, based on a commentary on the Citizenship Act, they lost Austrian citizenship at the point of resuming their British citizenship. There is too much at stake for me to risk this – it is not the Kavalierdelikt some think it is. See also my post on A quick primer about the Staatsbürgerschaftsgesetz 1985.