Can't Re-authenticate Additional User Account

Hi Guys, I've been setting up my father-in-laws ring system after they moved house and got things working on his phone fine. But I'm struggling to get things working on my mother-in-laws phone. When trying to log in from either a browser or the app using her account, it asks for a new authentication code and every time I generate a new one from he browser under father-in-laws Ring account, from the father-in-laws mobile using the Ring app, or using Google authenticate and his ring account. Every code fails. It's a Ring Video Doorbell 2. Any idea on what I can do to get this sorted, please? Many thanks in advance. Mike