What should my friend do about her visa and work permit situation in Thailand after resigning?

If she is on good terms with the employer she should ask them to extend the date when they cancel the work permit. Other than that she really has no option but to cancel her visa matching the day the work permit gets cancelled and leave that same day. She could also leave to a nearby country and come back visa exempt (assuming that's available)

Your extension is not automatically canceled when your work permit is canceled, but your extension based on employment is invalid when you are no longer meeting the requirements, which means when your work permit is canceled. She needs to go to the immigration office to cancel her extension too when her work permit is canceled. Both cancellations can be done in advance in most provinces.

Totally depends on her employer what date is her resignation effective date. She only can be in Thailand legally until that date.

No tourist visa or extension is available for non-immigrant type visa/extension holders. She can apply for an extension for any reason, but the application will be denied, then she can get a 7-day-to-leave-the-kingdom stamp. It is no longer extension, she definitely needs to leave until then. And land-border-bounce may not her option anymore once on this 7 days stamp. Also, this 7 days stamp will be started on the day of application, not added.

Maybe one of the easiest ways for her to stay in Thailand for a while is border-bounce when her current extension is canceled if her nationality allows it.

BUT it's her own decision how she wants to handle her own situation. Nobody in this group knows what her preferences or conditions exactly, any opinion by anyone other than her might not be applied to her in the right way.

1. she has to leave on the date of her resignation on paper

2. no

3. that depends on her employer how they manage the procedures

4. no extension for tourism is available for her