Hello SzeYong
You must buy the Swiss Travel Pass on her behalf with her e-mail, create an account for her before buying the pass, then buy the pass while authenticated.
Later, log in on the app using her account credentials.
Alternatively, you can buy her STP with your account, and log in on the SBB app on her phone using your account. An account can be used on three devices simultaneously. By doing so you both will see all tickets.
Best regards,
Roland