"Buddy not authorized" *and* "XMPP message delivery" errors

[Quinn Collard]

Trying to add a new Gtalk buddy and getting a "buddy not authorized" 
error. The help I'm finding is all about steps needed for the initial 
setup of Gtalk, like telling Google to allow less secure app 
connections, but I've already had the account set up for some time and 
am able to chat with other buddies--it's only this new one that I'm 
having trouble with.

My friend also did something on her end and then she was able to message 
me, but when I tried to message her back I got the following error:

XMPP Message Error
Message delivery to [redacted]/IOS95CA59E161 failed: (Code 503)

I'm using Pidgin 2.13.0 on Windows 10.