Earlier quoted context omitted.
I've done a similar thing with a few past employers after I've moved on. "Hey, can you explain this code to the new person?". In some cases, I'd be glad to do a 1 hour call or whatever. They frequently ask me to submit a bill for my time and I always refuse. I don't want to turn into some kind of staff augmentation and be at their beck and call "because they are paying". Way better to think of it as a gift and I get…
That just sounds like you're being taken for a ride
If I'm helping then I refuse any kind of money. I don't give guarantees or service like a business does, I don't want money like a business does. I do often ask that they try to learn from the experience, I'll gladly explain everything I'm doing.
The other option is I refuse, for whatever reason that is.
This keeps my choices those of a friend and my responsibilities those of a friend. I like keeping them that way.