I can't downgrade webview nor update it because I'm locked out of play store. What can I do?

My mom's phone (yes, I know what you're thinking :-) suffered from the webview update glitch yesterday. She thought that for whatever reasons it might have something to do with her credentials, so she set a new password from another device. I know about the fix for that glitch, but can't do much about it using the recommended methods because now she's locked out of her Google account on her phone. Obviously, trying to update her credentials on her phone must bring webview which doesn't work. It does sound like some kind of catch 22 that my very limited knowledge can't help me getting out of. Any help would greatly appreciated, thanks for your attention.