The problem comes when you have to actually edit Pepito’s name since you will have to modify it everywhere you have used it and not only in /users.
Firebase: the great, the meh, and the ugly
Pier Bover

Why use Pepito’s name when you can use the uid of that user as the author in the chat message? That way you only have to change the user’s name in /users, and since the uid is unique and never changes, you’ll only have to look up the user with that uid and get its name and display it. I just started to use Firebase recently and I’m not an expert in any way, but wouldn’t that just work for this case?