Made 2 test payments – one via Google Pay and one via SOFORT. Both went through fine, also received success emails from the site (not Stripe receipts though – it does not send emails in test mode).
The issue for your customer could be caused by errors on the page like I mentioned earlier. Open Chrome Dev Console (F12) and make sure there are no errors there (especially related to JavaScript and HTTPS). On some browsers\devices those errors could mess up payment process, which could cause various random issues (like Apple\Google Pay buttons not displayed, payment process stuck etc.).