Just leave them to suffer? Unless you need him as a reference for future jobs, I guess. Sorry he ruined your day, OP.

The English language has a great word that is just perfect in this situation: No. Google it!

You're not required to do it if you don't want to since you're no longer employed. It's ultimately up to you

I hope you go in. Let the new person call you if he needs. Be kind. There is so much hate in this world. Be above it.

I'd do as they say, OP. There could be consequences if you don't: *OP's ex-boss motions towards chair* "Take a seat. Listen, I'm really sorry to have to do this, but your performance has dramatically suffered since you left, so I'm afraid we're going to have to hire you." "What… I can't believe this. You can't HIRE ME! I've put in so much time since quitting!" "You have put in a lot of time, and makes me so sad to have to hire somebody who showed so much potential. Please pack up your things… and bring them here Monday morning, 8AM."

So now you can go to your old job as a freelance trainer and ask for MORE money!

Go ahead and tell them you'll come in, but as an outside consultant, for at least $100 per hour

I wouldn't waste your time unless your really depending on them for a reference and if you go in charge double!

You quit. You're done. You have zero obligation to go in and, depending on a reference or not, you should respectively say "I'm sorry but I'm unable to come in. I am no longer employed here and it is not my responsibility to teach them. I urge you to use the how-to guides I created last week for them to use. I wish you the best." FYI, usually, most places don't actually call your references. Sometimes they might but it's not really often.

So your reply was in the vein of "**** no."