Hallo DaveRgb
Yes, anyone can buy tickets at half price without a valid half fare card, as the card is not a prerequisite for the purchase of discounted tickets. You need it only while traveling with discounted tickets as a proof that you are entitled to do so.
So simply add her as additional passenger during the first ticket purchase process, making sure that ‘half fare’ is selected for both persons, so the correct ½ prices will be displayed and charged.
During a ticket control, she will need to present her phone as well though. (Or alternatively her swisspass card, assuming you mean the 1y valid half fare travelcard intended for residents)