Would she use one of those little password-keeper books? It’s not as secure as a password manager, but it might help get her self-sufficient.
You could start not knowing how to do things, give slower answers, just give bad customer service. Or ask her if whatever she’s trying to do can wait until she gets home to get computer.
I know the feeling of wanting to help, it’s part of why I became a librarian. I also know the pain of old folks coming in and asking the same questions. I had one lady, really sweet, that would come in and ask for the phone numbers to maybe 3 businesses a day. Like, we’d show her how to look it up, we’d walk her through it on a public terminal, she’d still ask us again the next day. It gets frustrating and you pick your battles.
At least I could go home after a shift and stop being the tech-knower. It doesn’t sound like you get to and that sucks.
Tell that to the grumpy old folks that sit at my library and grouse while waiting for their turn with the newspaper. I’ve seen queues 3 deep. No, they won’t share. Yes, they will spawn camp the newspaper rack.