Only one user cannot log into an app via Google ID Authentication
If it's only one user I'd say it's fairly safe to assume the problem has something to do with that user's credentials. It's hard to say without an error log but if it were me I'd first check to make sure the information the user is entering is the same as what oauth is expecting. Good luck and hope this helps!
At work we run a python application where users log in via their google account. 
One user gets an "Error logging in" message on any instance, this doesn't replicate on any other instance. 
The app was made by a third party and they can't tell us why this happens. Is there a debugging tool or something that comes with Google auth that could be used to trace where the failure is happening?
Thanks in advance. If any more technical details are needed please let me know. I'm not very familiar with how all this works.