You can check her in using the app and get the boarding pass. If you both use iPhones, you can text her the Apple Wallet (ne้ Passbook) boarding pass; she never has to use the app herself. She can click the Add button; once she's done that, it will show up on her lock screen. My spouse and I do this all the time; whichever of us happens to have made the reservation checks in, gets both boarding passes, and texts the relevant ones to the other. And FWIW, I have an up-to-date iOS and she has a several-year-old version on an even-older phone.

I don't know if Android has something similar if that's what you use.

And as others have said, she can certainly print out the paper boarding pass from a kiosk or an agent after being checked in online.