Hello TeddySCH
At the ticket control, you must be able to show the ticket (Day Pass for Friends) and the subscription (Half-Fare Card). Without a Half-Fare Card, the offer is not valid and you will pay a surcharge on the train. If you buy this day pass with your login on SBB Mobile, it will be visible in the app. Otherwise, you will receive the ticket by e-mail and you can show it on your cell phone using the PDF attached. The same applies to the purchase on the website, except that the ticket can not be loaded on the app even with login.
Have a nice day.
Greetings, VlaznimA.