Whenever she is online and wishes to transfer money to somewhere else - at some point in the transaction she has a to click a button on her PC and almost instantly her mobile phone 'dings' as it receives a text with a secure code from the internet bank that she then has to input to the PC in order to proceed.
Me the same and I don't need no stinkin' app. The code is a one-off, arrives within seconds, and as I'm waiting for it, it normally gets entered within a few seconds. Acceptable risk vs convenience as far as I'm concerned.