I doubt that this is app related, especially if it’s just one user on one device - my guess would be that it’s related to cookies (either a browser error or his settings). I’d try clearing the cookies and see if it helps. Also (and this may sound obvious) have you checked that he’s not using the browser in incognito mode?
An overeager app blocker could also be the culprit. If the other suggestions don’t work, ask him to disable any blockers for that url.